I can’t neccessarily offer any help but I can say I have had similar problems and have pretty much the same set up as you. I have the sapphire 4850 and 4GB of ddr2 RAM (1066). I bought/built the pc specifically for video editing and using the adobe production suite.
I have a 64 bit system and have tried using Vista Ultimate x64 AND Windows XP (x32) with SP2 and SP3. It’s been very frustrating. I have worked predominantly in Premiere Pro but did have crashes in AE as well. It seems any “big” job will crash the system. Which is absurd since this should be no problem from a pc with these specs.
I read some stuff on the net and I’m starting to think it might be related to the HDs or optical drives. I burned a dvd in encore last night and I did get some errors when I initially opened the drive door. I can’t tell if it’s a hardware or driver issue. I’d guess the latter since I switched out the video card (for an identical one) and the same thing happened. I think it’s the mobo/GPU/CPU combo that is the problem. So few people probably have pcs like this that it just hasn’t been discovered or addressed yet. Good luck.
It’s nearly impossible for me to convert my AVCHD to something else in Premiere Pro but with CS4, using dynamic link, I was able to export it to Encore and creat the DVD with no issue at all. Not that it helps you.