System crashes under stress

I have the ASUS P4C800-E Deluxe Motherboard and the Kingston Value Ram DDR400 (4GB)
The CPU is a Pentium 4 3.2Ghz and the video card is XFX GeForce 6800 Ultra.

About a month ago I started locking up during my online gaming session periodically and then with more frequency as time went on to where is was unbearable. The screen would pixilate mostly with green dots and some wavy lines after which it would freeze. The only way to fix it was to turn off the computer. If I don't play games that are graphics intensive then everything works fine.

I did some research and decided to try and save old faithful so I bought a new video card but that did not solve the problem. Then I bought a new power supply and still the same thing. I am guessing that my next step is replacing the memory but I fear that it still may not fix the problem and then I have a bunch of outdated parts which I will never use on a newer system.

Does anyone know if I should go ahead and replace the memory or is the issue related to the motherboard or CPU?