I have a Radeon x800GT graphics card in my computer at the moment and it seemed to work ok only lagging a little in games like the movies where my CPU was probably more the problem. But since I got my 21" CRT and started running all applications on 1600x1200 it has started crashing more and generally slowed down. Are these linked and would it be worth investing in a better graphics card like the GeForce 7800 series?
Running at 1600x1200 will cut your frame rates, it should not result in crashing. I think you have a heat issue. You need to get more air blowing through your pc, and especially over the graphics card.