I think my PSU (Corsair HX 520w) is not strong enough for my rig ... when I overclock much past 720ish my computer has the screen go black a time or two, or just locks while on the desktop, even though Rivatuner shows the temperatures as 50ish for my SLI pair. I have upgraded air cooling (Thermaltake ND1's). Also having it lock if I go into 3D when overclocked, though I can run the Crysis GPU bench over and over without issue at its 675/1950 "KO" speeds. Another thing that's making me think it's the PSU is that ATItool can heat the cards up to about 66, but it locks sometimes after 5-6 minutes, sometimes after 1-2, even when the temps aren't as high as it's gone before without locking.

My system is:

Q6600 @ 3.15, 1.53v
2x eVGA 8800GT KO 512MB in SLI
2 hard drives
eVGA 680i NF68-A1 SLI motherboard
4x1GB RAM sticks
7 120mm case fans, plus two 80ish-mm fans that the aftermarket coolers use
Optical drive
X-fi sound card

Not sure if it's just a really bad set of cards for overclocking, or if it's just too little power from my Corsair HX 520w PSU ... any thoughts? It seemed iffy in the beginning, now I'm thinking it actually isn't enough.