AFAIK 'any' 7600GT is SLI capable, now that they've fixed this in the driver you can use different brands too - although they will default to the capabilities of the slower card.
If I were you, I'd try and get a factory-overclocked version from the likes of evga or XFX.
Actually, if I were you I wouldn't buy a 7600GT, but thats not really helping your question
*cough...7900GT, X1900XT ftw...cough*
In my opinion they are the single-GPU solutions to go for if you want a balance of cost / performance.... X1900XT in particular I'd personally recommend.
If you go cheaper, then existing high-end games are gonna be pretty slow let alone upcoming ones. Of course you can spend more, but if you could afford a 7950GX2 you wouldn't be posting about 7600GT.