I have 2 7600gt and on 3dmark06 with one card I get around 3000 with 2 cards I get around 6000.
Don't know how well it applies to games but it doubles my 3dmark score. I really don't play many games that would push my cards hard enough to notice a big jump.
I would go for it. I got 2 7600gt a year ago for 80 bucks each and they sell on ebay for 80 bucks now so If I wanted to I could basically get my money back on them.
The main reason I went sli is that I wanted a extra computer for when my little brother visits I just take out one of my 7600gt's and put it in the spare computer and bam now I have 2 computers for a incest lan party....or a regular lan party if that suits you better.