If you had to buy now, you have no other choice than the GS!
Anyways, if the 9600GT is actually what it promises to be, it will outperform the GS by a bit. How much? We don't know. There are continually rumors about it getting delayed, recalled, etc. due to newfound problems.
To sum things up: If you're patient, wait and see what the 9600 truely has to offer vs. the GS. If you've got to get something now, get the GS.