SLi has come a long way, used to be that it had to be the same firmware/brand...
I wouldn't bother going for one with the extra VRAM, you won't benefit from it. SLi doesn't take memory capacity into consideration like the old 3DFX used to, it's purely GPU processing.
Anyway you can SLi any two cards as long as they are the same core/chip... so as long as they're both the 'GTS' you'll be fine
Edit: if you've already got a 1gb buy another one because in SLi, it'll cap you to 512mb of the newer card... thought it was vise versa