Of all the medium/higher-end GeForce 8/9 cards, one must remember that the old 8800GTS was, by a huge margin, the weakest; if memory serves, it was actually weaker than, say, the Radeon HD 2900XT and HD 3870, which really says something about it.
The 9600GT is a serious leap over it, and additionally, uses a smaller die (rather than the original huge G80) meaning it'll also be running cooler.
Of course, as also mentioned by Avenger_K, if you can afford it, you'd get much more for your own money if you went just a step higher, with the likes of a GeForce 8800GT or Radeon HD 4830; both represent close to the highest-end performance you can get with a single-GPU card currently, yet run well under $150US each. (and note: the 9800GT and 8800GT, performance-wise, are identical cards, with identical hardware setups and clock speeds)
when building my computer 4 days ago I was choosing between these two exact cards too! and read up on it and found out that the MSI 9600 GT OC was way better, although... now I am having power issues! it says I don't have sufficient enough power to power my videocard, I ordered a new psu.
Make sure your PSU is atleast 25 amps on one rail! (required is 26 amps)