Poor graphics performance/artifacting. PSU issue?

I just bought a new Asus 8600gt graphics card. Playing games with the card results in extremely poor performance compared to benchmarks I've seen in reviews, and in some games (stalker and bioshock with high level shaders enabled) artifacting also occurs (flickering black polygons etc.). I've checked the load temperature of the card and it doesn't go above 55 degrees C, so it seems overheating is not an issue. The card is also running at the stock clock settings. The latest Nvidia beta drivers (163.44) are installed (and I've also tried the non betas, 162.18, made no difference), I ran driver cleaner in safe mode and I've tried reseating the card.

One possible cause I can think of is my PSU. The card is in a dell dimension 4700 desktop computer, and the psu is 305W
(specifications here: http://support.dell.com/support/ed [...] #wp1053452).
However, I don't seen any of the usual PSU issues (random resets etc.), just this graphics issue. I've also seen on several forums that people have been running the same card with the same dell psu without any issues. Is it possible that the psu isn't supplying enough power to the card, causing the poor performance?

If it helps, my previous graphics card was an x300SE, and I had no such problems with it.

Thanks .