PSU won't drop your score... Does it pick your GTX 560 and not integrated graphics (if your i3 has it) maybe set something in your Nvidia Config or something? But besides that.. could you perhaps be more specific about your rig?
Intel Core i3 xxxx?
and if i were u, i would always keep Windows 7 64 Bit installed if it's supported.
Is it only 3dmark what's giving a bad score? or is every game performing lower? I think that 3Dmark is testing it with your integrated graphics instead of your GTX560.. In Nvidia's Config panel you might find something to lock the graphics engine on GTX560 or something, otherwise maybe in PowerManagement.. I'm not really sure about it tho..
Is the performance worse than your GTX285 or just less of an improvement as espected... Maybe it's a CPU bottleneck?
Well i'm not an expert.. but it might be that it's more of a CPU bottleneck then.. If u want to be sure u need to ask an expert.. but otherwise i would upgrade my CPU i guess..
Sorry did know that but typed wrong card on accident
Re-seat your card and power connectors and disable the onboard graphics through the BIOS.
gtx560 is a mid-range card and you do have an older dual-core CPU, so ultra settings is an unrealistic expectation in demanding games, but you shouldn't be getting less performance with a better card. That doesn't point to a CPU bottleneck, that points to a hardware or driver issue.