I was pretty sure the problem was overheating, as 9800gx2 does that. A few days ago it was over 100ºC... so today before playing games I've opened the card, removed from its case, cleaned it up (superficially - as I didn't have all tools needed), didn't mount the case back as I read online it helps and I confirm (the log proves it too), temperature wasn't an issue at the crash moment.
Today I've used Everest and recorded a log (attached here). It's a long log, but if you scroll to the end you'll find 2 lines highlighted in red. That was the exact time when the game crashed (BF BC2). I was in game when stuttering started, video modes changed, screen went black, screen went white... I had game sound but no video. I've alt-tab, started Task Manager and ended game process so sound stopped and my machine was 'operational' again.
I've never heard of this Prime95 Torture test, so I'll look for it later on when I get home and test it...
About the video card being deffective... I'm not saying you're wrong but there are times no issues occur at all... I mean, hours of fun online gaming (BC2, L4D2, GTA IV, etc), while there are times after some time playing it crashes. (in the log you can see it was less then 50 minutes). I don't think I've ever come accross something like that...
I forgot to mention the PSU is a Thermaltake ATX Toughpower 1200W W0133RU and as it can be observed in the log file the info provided by the components shows stability.
Problem fixed... I've changed drivers (Nvidia has just released new ones) and instead of letting the driver choose which GPU should be used for PhysX I've manually forced one and it stopped crashing as well as fps is a little higher now.
Not really sure what's the reason for that but now my system is operating regularly after hours of gaming. I've also used the Furmark which ran just fine but when temperature reached 95º I've decided it was enough.
So... for those with X2 boards presenting instability I'd suggest manually forcing one of the GPUs to do the PhysX part.