Dont do it , dont waste your money, 2 8800GT outperform a single 8800GTX in most games but not all (if u want , i will give the links of it)
also when it comes to very high resolutions, a single 8800GTX does better(again i have the benchmarks ) also 2 8800GT will consume more power and also are more hotter than single 8800GTX
SO DON'T DO IT, seriously what game on earth u have that u cant max it out with a 8800GTX? even in Crysis (with my setup, but VGA @ stock) i could play it with everything @ high(using XP 64) with 1680x1050 resolution and it mostly is between 26-50(FPS) and it runs very smooth, no lags @ all
Seriously there isnt any game that i cant max out with it
AGAIN DONT DO IT
A- waste of money due to shotty SLI drivers ie performance will either be same,or worse, very few that its better..at least not worth it imo; Yes there are some cases SLI is like 20-40fps better, but if your getting over 60fps anyways its kinda a waste..especially when you take into account B and C.
B- something new coming out in jan/feb (Early Q1)
C- Good luck finding 8800GTs... most are 300 bucks thats 600 for 2...no 1 is going to pay 600 for a used 8800GTX since brand new 1 is 525 bucks (EVGA 2..most expensive 2 lol)