I agree, going from a 7800GT to a 8600 anything, won't yield a massive improvement.
My brother in law has a 7800GT (256mb) card as well, and they are still very good cards in every game except Crysis lol.
Heck I think even going to a 3850HD might be a little bit too modest considering the card you have! If I were you, I'd go for the 3870 or 8800GT, or wait for something better yet like the 9800s in February (hopefully).
Man..... If only I had at least a 7800GT! =0