pick out what games you want to play, and then look at the interactive vga charts. Some games work better in sli then others. depending on the game, you can get 8%t 80% improvement
stay away from he 7950... personally, i think its a fad card, and not very useful
also, as cards drop in price, they also drop from shelves. I bought a 7800gt when they first came out, expecting to upgrade to sli (even bought a mobo with 16x sli). but it is very difficult to find a card with that chip, and i cant find my exact card anywhere.