Humm...Have you made sure that your two sticks of ram are in the right pair of slots?
Also, NVidia cards really eat up the watts on a PSU. Methinks your PSU ain't supplying enough juice for your card.
Actually that PSU is enough for that setup, GPU wise anyways. We use it in most of our basic builds at my work and I have put even a GTS450 in a system with it and it can run the WEI without an issue. Its when you move beyond that that it needs more.
Still it could be a PSU issue, possibly a bad one, but it being under powered, I doubt. Unless the CPU is using all the PSU at the time but I don't see why it would since the DX assessment is GPU based, not CPU.