I don't need to at the moment my 460 does me wonders on all games, but in the long run should I upgrade to a whole new gpu or just sli another 460?
Depends on your budget, if you can afford a new single card then go for it, could probably make some cash back selling the 460. If not then SLI would also give you great performance, in some cases better than a 580, for a lot less.
For the long run I would be looking at a single High end card (such as the HD6970 / GTX 570 replacement) but if you want the performance now you can't go wrong with GTX460SLI. I used to run that exact setup (GTX460 1GB SLI overclocked @ 850/1700/4200) and in general the performance was fantastic. Roughly it offered HD5970 performance with better scaling on the whole.
I agree but what case do you have? GTX460's can be hot little beasts! Plus all that extra power needed! TBH i'd sell the 460 and grab a hd6950, flash it to hd6970....but im biased as I did this