Hmm, if it occurs when your exiting a program such as quake that would use all the memory anyway, the chances are it might not be the memory as it would more likely crash at any point when that 'faulty' bit of memory gets used. I'm guessing it's more a driver problem.
Any chance next time you get a blue screen of death you can note down what module or any file names that it may mention. We can then take a guess at whats busy when it crashes.
In the meantime, to rule out memory as the problem (as I'm not saying it definately isn't that) try a memory tester. Theres plenty out there, but heres a link to Memtest86
Just thought of something else too. Give me your full system specs, specifically your motherboard version and BIOS version. Try and lower your Memory Bus Speed, as in the case of using 4 x1GB Dimms, almost all motherboards won't allow you to run them at there top rated speed in this setup.