After reading this, and experiencing the full gfx fan and no display problem (now changed gfx card tho), I checked my OC and memory timings and they were all default again! Seems like the BIOS has reset. This has only happened since I started playing GTA4. However, it could be new nvidia drivers, as i've changed them a lot recently, or a windows update? I can't believe it could be GTA unless someone has a decent theory on how this could happen?
Thinking about it, I had a random black screen in GTA4 that required a hard power off, and thats when I had the booting problem.
This is probably why my frames are the same in GTA4 since installing a GTX280 (albeit with higher settings), as i've lost the CPU OC (i'm probably mostly CPU limited in this game apparently).
Anyone else with this issue then?