You need to check if your power supply is enough to power an overclocked CPU and a 3850.
Looking at 3850 prices, it probably wouldn't cost much more to buy a new motherboard with PCI-e, the $12 after rebate ddr2 and a 4670 (low power requirements here), which according to
anand's article does very well compared to the 3850. That way, you can upgrade to Core 2 next when your CPU becomes inadequate.
Edit:
cheapest 3850 agp - $130 after rebate
upgrade:
2gb ddr2 - $12 after rebate
link
4670 - $70 after rebate
link
cheap motherboard - $40
link
Upgrading actually costs slightly less
It would be way less if you skip on the 4670 right now and get the $25 ram + 8600gts deal currently offered. Then you can upgrade the video card later if you feel it's inadequate.