Why not sell your 8800GTS on ebay or somewhere and get a gtx260? If you were looking at spending $80 for a second card, you should be able to sell your for the same amount at that puts you in the same neighborhood as a 260. I just sold an 8800gtx and got enough to buy a new GTX260 and had some left over.
That was me >< But if you can afford it, i'd suggest selling your current gts and buying a 260. It'll come out to around the same cost, but the 260 will perform better.