Hi
I have the same problem as you with my setup
AMD Althon 64 X2 3800+
Plait 630 2GB GDDR3
RAM 6GB , useable 4GB (wonder why on 64bit it still have limit )
The solution is about the PSU (Power Supply Unit , ) can't have enough power to power the CPU
1. too many fans installed in the Case
2.Over clocking
Try getting a better PSU , get a 600W and branded type as it for sure it will fix your problem of screen going black
And also you should upgrade your RAM to 4GB at least as most games are hungry memory monster xD