well obviously 2 cards is going to increase performance, but only by around 50% which makes SLI really only an option for those with money to burn
with the older GTS chips (G90) the gt is actually faster, but the old GTS costs more because the retailers are still trying to shift the old GTS's to uninformed consumers
The new 512mb GTS is a totally new chip (G92) to the old ones and is much better than the old one, but still I wouldn't recommend it over the GT you already have due to the extra cost for not much performance gain, It has more stream processors and slightly higher clocks- oh and more memory bandwidth, but I think its priced a bit too high at the moment- If only you could pair up the GT and a GTS in SLI. Seeing as you already have a GT I wouldnt say you should get a totally new card which isnt that much better, but if you were upgrading from a 7 series the GTS would be the obvious choice
I would say stick with the GT, get another if you're having problems running games- but do you really need the extra GPU for 1080p? My one GTS (640mb) is battering everything I can throw at it on 1650x1050
what else are you running in your rig?
Have you tried over clocking the GT- apparently they clock up pretty well (if you can keep them cool that is)
that said seeing as the GTS runs cooler than the GT you could overclock it more
they say you should never upgrade within a series- and to some extent I would say that's true, I know i'm sticking with my 8800gts until the 9800's come out, i'll probably SLI them though if I can pick one up fairly cheap next year
