I was in a similar situation as you. I have a Q6600 @ 3.2GHz and had a 2900XT video card (similar performance to yours). I didn't want to spend a ton of money on the 5xxx cards, so I went with a 4870 for $85 off the FS/FT forums. It increased my GPU score in 3DMark Vantage from 3,900 to 9,200 at stock clocks. I overclocked it a little to hit about 10,300.
Definitely a HUGE difference. Real-game benchmarks put the performance increase at about 110% (i.e. more than double the performance) with stock clocks. A 5850 would be about 55% faster than the 4870 I have; however, it would cost 200% more.
For me, the 4870 is a card to hold me over until the next generation of cards come out. Hopefully the price/performance will be better than the 5xxx generation, or else I'll probably stick with my 4870 for quite a while.