SLI is having 2 cards on one motherboard (Scaleable Link Interface, i think is what it's called). You have to have the same graphics card in order to SLI them. Right now, SLI really won't do you much of any good. The performance increase compared to the price increase of SLI isnt worth it. I'd get 1 7800GTX 512mb, it will last you for years, and when it gets old, just get another one. By that time, more games will have better support for SLI and you will see a much greater performance increase by having 2 cards.
AMD 64 3200+ Newcastle@ 2.2Ghz
1GB (1x1GB) OCZ Performance Series
ATI X700 Pro 256MB OC\'d 475/525
OCZ Modstream 520W
Samsung 914v 19\" 12ms LCD
SB X-Fi w/ Logitech Z 5300e