Not worth it IMHO.
The gts will get a few more fps in many cases... however the THG benches say the 7950 IS better overall :?
See how much u can sell ur card for. If u dont have to spend more than $30 on a gts it MAY be worth it.
But id say just wait till the 89xx come out.
a) they will push the price of the 88xx down
b) 89xx will have a nice little fps increase over the 88xx