Not without researching, so go looking, cause that is important.
Its best not to plan mate really, just turn up in a town, walk to the hostel/hotel and get a room. Things are less exciting when you plan them to death, and it just puts you off going in some ways.
Don't buy all your guidebooks in UK, buy maximum of your first 2 countries. You can buy out there easilly (no cheaper though, unless you get second hand). I would spend more time in Thailand & less in Cambodia, there isn't as much to do in Cambodia. My Cambodian guidebook was 3 times smaller than my Thai one :-) I had enough of Cambodia after 10 days actually, got my wallet stolen on the last night too. Its just too poor, and theres no eminities or infrastructure, its much harder travelling there. Its up to you ofcourse, this is just how I felt. Don't worry though, if you do/dont like it you can always spent more/less time there, thats the great thing about not planning too much. It wouldn't surprise me if you spend much longer than you mean to in Thailand, it happens to most travellers to be honest, especially with it being your first location. The people are just so nice, and everything is set up for travellers there its easy. Transport, acommodation is a breeze and there are 7-11 all over the place to buy drinks & food. Thailands the kind of place where you get somewhere and plan to spend a few nights, but end up spending a few weeks on a beach.
2 month in China is ample time, you could do a hell of alot in that time. Look on the map, do some net reading, have an idea of where you want to go, then leave it at that, and wait till your in Vietnam. Then buy a guidebook & start planning.
BUY A GUIDEBOOK!! Most of the stuff I & others have said is written very clearly in there.