Generally speaking the GTS 250 will DESTROY the 7600GT. However, in your case, the 3800+ is probably going to hold you back a bit. I'm not so sure you'd be able to get 100% performance out of the GTS 250 anyhow.
If you have an AM2 motherboard, I'd suggest upgrading to a newer AMD CPU. If you've got the older AMD socket 939 motherboard type, I'd say you're due for an entire upgrade before you'll see any worthwhile gains.