Destru Avedon

I had decent performance with SL for years (8800 GS, Dual core e4500 CPU)...until I tried overclocking my GPU. The graphics became checkerboarded, probably due to overheating, and I eventually got a new GPU. My set-up still gives reasonable scores on benchmarking software, but SL is miserable. I'm stuck at <10 FPS in most sims, unless I'm somewhere bland and deserted, and then it'll go to 30-ish. Any ideas? Time for a new CPU, motherboard?