Just adding my voice to the cpu not being the bottleneck. I've got a Q6600 with a 4870, and I'm getting way more FPS than that. A quick google suggest they might have skimped a little on the mobo, but I don't think it's anything catastrophic there. Did you get decent performance out of the 8500 before it died? If so, I would:
1) Make sure you've uninstalled everything correctly for the old nVidia card.
2) Make sure the catalyst drivers for the new card are up to date.
3) Check that your motherboard can support the card, and does not need a BIOS update.
I'd get the issues with your current rig sorted before overclocking, as you should be getting better performance than that. If you want to overclock, get your hands on an aftermarket cooler. There are quite a few out there that will fit the bill. I've got the Arctic 7 Freezer Pro, which is cheap, and keeps my Q6600 happy at 3.2. Let us know how you get on.