Something that hasnt been mentioned yet is that you are running 64bit windows. 64bit is unstable. Alot more so than 32bit. Before doing anything else, I would recommend downgrading, because there is really no other advantage to 64bit other than the amount of memory the OS can recognize. 64bit is not faster it is just bigger, and more unstable.
If you can, downgrade, if you cannot... Im going to say your problem is either overheating or a bad part.
I know you were already advised to run a memtest but the one you were advised to is unreliable (in my oppinion). Ultimate boot cd is much better and it has HDD RAM and CPU tests. (http://www.ultimatebootcd.com/download.html
UBCD is a fantastic diag tool. Download the ISO and burn it to a cd.
You must first run the HDD test, and then the RAM test. This is because if the RAM test comes up bad it may be the HDD's fault. If those tests are good I would then run the CPU test.
If all of those components are testing good, I would then direct my attention to the board. Check for popped or bulging caps. Also I think UBCD has a diag for the chipset, you might try that.
If these lockups are happening during gaming sessions, I would look at the game. If you are playing crysis, I will wager that might be the problem. the GTX260 might be submiting.
The main cause of lockups, however, is overheating and just because you have 4 fans doesnt necessarily mean you will not have heat problems. I would get a thermal sensoring device (30-50 bucks) and check every square inch of your pc. check the vcard cpu hdd and ram. Something might be getting too hot.
Also, try increasing your fan speed. Your 4 fans wont do much good if they are operating at 50%. turn them up to 90-100% and see if that works.