Computer issues

Hey guys.
For a pretty long time, i have never had a very stable computer.
I relatively frequently experience problems with my computer including:
BSOD: pagefault_in_nonpaged_area
BSOD: pfn_list_corrupt
superfetch has stopped working
host processes for windows has stopped working
far cry 2 unexpectedly crashes, no error message, it just closes
zune stopped working
zune unexpectly stops working, no error message
hl2.exe has stopped working
fallout3.exe has stopped working

These are some, but not all of the errors i have experienced. I have attempted to fix these by replacing my MSI 650i (open box) mobo, and 8600 GTS graphics card with a new EVGA 750i FTW mobo and 9800GTX+. Although the graphics card improved performance, i still consistently got errors. I bought a new hard drive and operating system, which i clean installed several times, but to no avail. I am now considering replacing my memory or cpu. I have also tried memtest, but I am unfamiliar with the program so it passed each stick once, and they passed, but I did not run it long enough to pass each more than once.

SYSTEM SPECS:
EVGA 750i FTW mainboard
EVGA 9800GTX+ 512 MB video card
3 GB OCZ Gold 800Mhz (2x512MB + 2x1GB)
500 watt SLI ready LOGYS power supply
E4300 1.8 ghz core 2 duo cpu