After looking through your previous issues on your other threads, one of the things I can suggest is to try pulling one of your video cards, deleting the old nVidia data (go to C:/nVidia and delete the whole folder), download the newest drivers, install, and reboot. See if that helps.
If this was already tried, I would suggest backing off ALL overclocks, and see if that helps. If this doesn't help, swap around your RAM sticks into different slots. Still nada? You have already tried a bunch of other common troubleshooting techniques, including a fresh install, so I suggest a couple of things that may help. First, if you are using Vista, stop doing so. Toss Vista and install XP. Only way to do this since Vista is already there is to get a 2nd drive or partition and install XP to that empty drive/partition, move all of your important things over to the XP drive/partition, and format the Vista drive/partition to completely rid yourself of it. XP is so much easier to deal with than Vista, and when Vista acts up, it acts up HARD.
If after this the problem still persists, then it has to be hardware. You said that these issues started after introducing themselves after adding another 2 gigs, and I'm sure that since you made this connection that you have tried things without the extra 2 gigs. Your next thing would be to swap motherboards. I know I know, that's a hell of a procedure but at this point you have pretty much exhausted all other troubleshooting tasks. I'm sorry that it has come to this, but repeated BSODs are going to either be software or hardware, and a fresh install of XP, with continued BSODs, means your hardware is crapping out. PSU dying would cause the system to just shut down or freeze, but with the mobo, it can be slow and painful.
Before you do this, also try to swap out other hardware, like video card and sound card. Get a cheapy vid card for troubleshooting purposes and a cheapy sound card for the same reason. If you find that the replacement of either the vid card or sound card resolves the constant BSODs, then the problem is whichever hardware piece was replaced.
If you do replace both the vid card and sound card, and the BSODs are still there, it is definitely the mobo.
I hope this has helped.