It really depends on the amount of RAM on either card.
The 8600GTS itself is somewhat faster than a 8600GT (540/1190/700 Mhz vs 675/1450/1000Mhz, same G84 chip).
If, however, your current card is a 512Mb variant and the new 8600GTS is a 256Mb variant it may very well be slower.
If they both have the same amount of RAM or the GTS has more and it is free, defiantly pick it up.
If you have to pay anything for it, save you $ for a 4670 as it is roughly twice as fast as either.