I think you should stick with just one card, if you don't intend to game at a resolution of 2560 or something. The cost of a sli system is just not worth it. Don't get the G80 chip (8800GTX and 8800 ultra) now, its more than a year old now, get the 8800GT if you can get your hands on one, and series 9 is just around the corner.
But i need the top end / best video card, since I'm planning to get a top gaming pc. So then should i get 2x 8800gt? or a 8800gt sli?
Also what if i have the geforce 7 series and bought the 8 series. Would it read the higher graphic card (series 8) or would it combine the 7 and 8 and make it even better?
Also what if i have the geforce 7 series and bought the 8 series. Would it read the higher graphic card (series 8) or would it combine the 7 and 8 and make it even better?
I think none of them would work, you would have to install only the one you're going to use..
And about having an SLI configuration, I wouldn't do it because you pay for 2 cards but do NOT get double speed, at most you could get is around 50% I reckon so definitely not worth it.
My pick would be one 8800GT, if you have a nice CPU and RAM the 8800GT is enough to be considered a TOP gaming rig.