Various Blue Screens - Win 7, self built system

I've been getting blue screens for awhile now, but they seem to be becoming more frequent, to the point where I can't really deal with it anymore. I built the system myself about a year ago, and the blue screens were few and far between at first.

Typically they are Page_fault_in_nonpaged_area, or Driver_IRQL_Less_or_not_less_equal errors, but every now and then I'll get a video memory manager crash. They happen mostly when I'm running a game, or have multiple videos open. Windows always boots right back up cleanly, and runs fine until the next error occurs.

Sometimes the problem doesn't get as bad as a full system crash, and something smaller will occur, like a game will lock up and kick me back to the desktop, or the flash plugin will crash while I'm browsing and I'll have to reload any videos i was watching.

I'm running the Win7 64 bit Ultimate Release client right now, but when i first built the system I was on vista. The problems have been the same on both OS'es. Common sense and a few quick google searches have suggested that it's either my RAM or my graphics card, so for RAM i'm running 2x2gb sticks of OCZ Reaper DDR2 1066 Dual Channel, and my graphics card is an Asus EAH4850 Radeon PCI-E with 512mb.

Thanks ahead of time for any help you can give me.

I'll give memtest a shot tonight. A detail I forgot to mention: Newer games like Borderlands and Left 4 Dead cause the crashes fairly quickly, but I can play older games like World of Warcraft for quite a while before I get any trouble.