Well my CPU is an Athlon 64 3800+ Venice (single core).
I currently have it modestly overclocked to 2.5Ghz, however if I go much higher than that then my system loses stability, even if I pep up the voltage a bit. It's certainly not a heat issue, I don't think I've ever seen my CPU go much over about 50C, but I have been running 1 x 512MB + 2 x 256MB PC2700 RAM and I wondered if that might of been connected with the poor level of overclocking I've been achieving.
My upgrade wasn't really so much for speed anyway, it was in preperation for BF2142, I could see I needed 2GB so I figured while i was at it I'd go up to PC3200 (which I know my board can run).
I got myself a Geforce 7900GS as well, but I can clearly see that I'm not getting as much out of it as I should, because my CPU is a bit of a bottleneck, which is why I'm now interested in trying to push it a little further.