The reason I got thinking about the PSU, is because yesterday I starting reading up on which PSU to upgrade to as Im going to order a second 7800GT next week . Currently Im using a el chepo Q-Tec 550W, but what it states and what it is are two different things. Although it has served me well over the last couple of years, I know it will not be enough for SLi, and maybe holding me back on my overclocking of my cpu.
Before my upgrade I had a Barton 2500+ @ 3000+ with a 5900XT, and I too had a couple black screens, but I didnt bother looking as to why as it may have only happened less than say 10 x over a few weeks, and only when I was away from my pc.
I found some interesting reading on PSUs over at the overclockers.co.uk forums
Im not saying it is your PSU, but it would be worth looking into.