uhh, your PC is a card? haha
EDIT - between those cards, i would go with the
but the 8600 doesnt really have a good chip, i think there was a post saying why its bad to own an 8600
if you dont mind having your first card as an recertified, you can get the EVGA GeForce 8800 GTS Video Card - 320 MB GDDR3 for $89.99.
though, i think you might have to buy a new PSU since those brand computers dont usually come with a good PSU.
the bad thing about that card is that it has the old chip, so its a GEN1 of the 8800 series, and most people have the GEN2 of the 8800 series, which performed better. I think an a GEN2 8800GT can outperform an GEN1 of 8800GTS.
But eitherway, it performs better than the 8600GT/GTS.