You need to use the same card, as the one that's already in your PC.You can, using the latest drivers
use a card from a different manufacturer as long as its the same model,ie another 6600gt.It is possible
by using some downloaded software to use a 256 mb
6600gt with yours,but it would run at the slower speed 128mb,so there's not much point.
Also if you're going down the sli route make sure you're power supply is powerful enough to drive two cards.
In my experience sli is only worth it at higher resolutions, if you're monitor is above say 22 inches.
Your best bet is probably to get the best single card
you can afford.