I recently upgraded from a pentium 4 to a fairly inexpensive dual core Intel. I do notice the difference. I think most folks are happy with any dual core that fits his or her budget. Check your motherboard manual for compatability.

You're right, NVidia 8800 in some variation is everyone's favorite right now. 8600 seems to be a scaled down version for bargin shoppers (like me). I haven't really put my 8600 to the test yet, but I've read it will at least play even the most punishing games. For uninhibited, smooth performance, I think you'll need to spend the extra money on an 8800.