prozac26 makes a very good point!!
You need a decent PSU because the 8800GTX has two power connectors instead of the usual one!!
If you go for two 8800 GTX's then you need a SERIOUS power supply!!! Between 800watts to maybe 1Kw!!
Also, your mobo will have to support SLI if want two cards!!
If you have an ageing mobo and RAM etc then get 1 card and spend the rest on a good mobo, processor and RAM
The 8800GTX on its own is seriously powerful though!!
It works best on the highest resolutions so I hope you have chosen your monitor well!! (if your new monitor's highest resolution is 1280 by 1024 then you are totally wasting your cash)!!
Post your specs and we can guide you!!