Bus from PP to HCMC: From Capitol GH $6 (I think also from Narin's GH). Arrives at Sinh Cafe in HCMC.

HCMC
Accommodation: 265 is quite nice, I think $7 single, $3 dorm, including breakfast. Dorm is on the 7th floor, though. Can't really recommend anything.
International phone calls: Cheapest at post office.

Mekong delta:
Tour: Most people take organized tours to the Mekong delta, they should be around $7/1 day, $20/2 days.
Busses: Are ok, I've seen worse. They're a bit crowded, stop a lot on the way, but they get there. There are lots of busses all day long between most towns in the delta, and between them and HCMC at least once a day.
Prices are very low, about $1 for short rides, bus from Ha Tien to HCMC cost 42,000D.
From HCMC to the Delta: I took a bus from bus station in Cholon to My Tho which, according to others, is a shitty place. I moved straight on to Ben Tre across the river.
Ben Tre: Town isn't much, has a nice market. Across the bridge there are very beautiful villages within dense orchards and an extensive canal system. You can get a boat ride for about 20,000D/hour. Stayed at Hung Voung on the riverside, 100,000D single, 130,000D double, negotiable. I've seen better places. There should be a cheap guesthouse somewhere, but it's far from the river.
Road to Vinh Long: Took a motorbike from there to Cai Mon, road goes through beautiful country. Took it further all the way to the ferry to Vinh Long. About 60,000D.
Tra Vinh: From Vinh Long took bus to Tra Vinh. Stayed at Thanh Tra on the north end of Pham Thai Boung st. - really nice rooms - 110,000D single (?).
There's a pagoda called Chua Hang which isn't so impressive, but had a nice chat with the monks.
Can Tho: Stayed at Hien GH, which is really a family stay. They're at 118/10 Phan Dinh Phung st., $4. Can arrange boat rides to see floating market and canals for 20,000D/hour. There's internet access somewhere for 400D/minute, ask someone.
Rach Gia: Stayed at Binh Minh on Ham Nghi st., 50,000D single, not bad.
Phu Quoc Island: Really beautiful island with excellent beaches. Took a boat from Rach Gia at 9am, costs 65,000D, takes 8 hours and really inconvenient, unless you're lucky as I was and you catch the fast boat that takes 6 hours and is really pleasant.
Stayed on the island at Kim Linh near Duong Dong. Really nice place, about 120,000D double, usually full, so you can try to get a bed in their improvised dorm for 30,000D. Very beautiful beaches, and they have good food. Can rent a scooter for $5/day and see the island, and some amazing secluded beaches. Took the boat back to the mainland to Ha Tien, about the same price, takes about 5 hours. Leaves at around 10am.
Ha Tien: Stayed at Dong Ho just in front of the bridge, not bad, 50,000D singe shared bathroom, 70,000D with bathroom. Can rent bicycle and see the countryside (though the bike I got was in very bad condition) for about 20,000D/day.

Visa: Can be arranged with travel agent for 1000B.
Transport from Bangkok-Siem Reap: Some travel agents in Khao San organise this overland trip, though it's more expensive. I took the train to the border, and was lucky enough to meet one of these backpacker's pickups that was happy to put me in the back for $5 with only 4 others, which was quite convenient (the others of course payed more).

Siem Reap: Heaps of guesthouses, if coming by backpackers' pickup truck, most likely to be dropped off near a guesthouse. I stayed at Green Park GH at #182 for s$3, d$4, very clean and friendly. Moto for $6 a day, extra $6 to go to Banteay Srei (which is beautiful, and the road there going through villages is also very beautiful), and $2 (?) extra for Phnom Krom, which is a temple on a hill near Lake Tonle Sap (the temple isn't worth it but there's a nice view). Internet seems to be now only available in a place called abc (don't remember where, it's where the locals use computers), very expensive - $3/5min, 13/hour.

Boat to Battambang is highly recommended, as it goes through BEAUTIFUL lake and river with stilthouses and boat houses and it's very interesting to see people living in the middle of the lake... ride costs $13. Sorry, can't remember the name of the place I stayed at in Battambang, there are a few ok places near the market. Got moto to take me around to see the countryside for $5. Very beautiful. Some hills (a rare sight in Cambodia) with monasteries.

Pickup truck to Phnom Penh: $5 in the back (maybe I was ripped off though), hard road, familiar scenery of rice fields and villages...

Phnom Penh:
Accommodation: May I add to the recommendation to GH #9 - liked it very much, really nice atmosphere. Food not bad, a bit expensive, there's a place on road 93 not far called Nga's Cafe that has good and cheaper food. Internet: there's a travel agent on Monivong Blvd south from de Gaul Blvd that has internet for $4/hour, can't remember the name (I'm not being very helpful, am I?), and there's also a place called FCCC on the riverfront near the museum for $5/hour or $4.5/hour during "happy hour"

8am-noon.Kampong Cham:
Shared taxi from central market in PP for 6,000R. Not very interesting place. Stayed at very clean and friendly guesthouse called Camnor Tunle for $3 single - walk towards the Mekong and turn right, before the bridge that is under construction.Boat from Kampong Cham to Kratie - 15,000R, about 4 hours.

Boat from Kratie to Stung Treng - 25,000R, about 6 hours. First part is boring - Mekong is wide and muddy. North from Kratie it is full of rapids and eddies and whirlpools, small islets with dense vegetation and wild jungle on the banks

Stung Treng: - VERY beautiful. I stayed at Sekong GH for $3 single, which was ok, not the cleanest though. Other guesthouses I've been told of - Star GH in Kratie and Amatrax GH in Stung Treng. Also heard there's a place where you can spot Irrawady dolphins in Kratie.

Stung Treng to Ban Lung: I took a 4x4 pickup, the center of the Rattanakiri province. 40,000R in the front or 35,000R in the back (you'd have to be insane to go in the back!) on a VERY VERY VERY bad road, far worse than the road from Poipet to SR! I don't think this journey is possible in the wet season. Road goes through very beautiful scenery and takes at least 10 hours (depending on how many times you get stuck on the
way...). Ask how many cars are going, if only one goes and it breaks down, you're stuck in the middle for the night! And it happens.

A word about Rattanakiri - This place is so wonderful! Both laid-back and adventurous. Hills covered with jungle with waterfalls and hill tribes and beautiful sunsets.

In Ban Lung, you're most likely to encounter Mrs. Kim as you arrive (no matter how you arrive - road or air), which owns and runs the wonderful Mountain GH and Mountain GH 2. Really nice and quiet places, rents out motorbikes, Mountain GH 2 also has a good restaurant. Bargain for the prices for accommodation and bikes (I swore not to tell how much I paid...). Also gives a nice map of the area with all the attractions. There are two very beautiful waterfalls, a volcanic lake, and lots of hill tribe villages around, and everything up there is very beautiful, very different from the rest of Cambodia (which is flat and irrigated), it's all hills and wild jungle.I decided to cough up a few extra dollars and fly back to PP for $49 + $4 tax (from PP the tax is $10).

Phnom Penh:
Minibus from PP to Saigon can be arranged with Capitol GH or Narin's GH for $6.
Visa for Vietnam can be arranged in many places for $35-$40, takes 4 days.