Which 8800GTS? There is two; G80(640Mb) and G92(512Mb), they will not SLI together.
This should be faster and is under $300;http://www.newegg.com/Product/Product.aspx?Item=N82E16814102813
But, two words of caution; This card is over 11" (28.5 CM) long and can be quite noisy.
PS if you have a G80 8800 GTS I happen to have one going (sort of) spare, which, spookily enough, is also a BFG OC card:) Post if you`re interested.