If the game even supports SLi. I learned that the hard way on my old rig with 2 x 7600gs. Ran great on games that supported sli, ran ok on games that didn't.
And ya, its not a perfect 100% gain, but you will notice a difference. Just depends on whether the cost is worth it to get 2 or to get one nice card. After dealing with sli id say save for the better card if your looking at sli. Or as said above, go EVGA and step up to the better card when you have the money.
The only reason I went sli was because I got each 7600 for right under 40$ brand new well over a year ago after rebate. It worked great on the smaller monitor, but with the new 22" it just wasn't enough juice.