I dont think it can handle SLI (dunno how many amps on the 12v rails.).
Now, id be thinking of getting 560 ti. You have 2 options here: reference card and later mayb buy a aftermarket cooler (most expensive but best option), or a decent brand, but you probably wont be able to change the cooler.
Now, if u are planning to upgrade your computer, a 6950 2 gb might be the better option, assuming you can afford it (775 socket cpus have a huge problem with physx games, but nvidia gpus can take care of that problem, thats what eventually convinced me to go nvidia).
I have 560 Ti paired with a q6600 at 3GHz wich i guess performes close to your dual core, and im happy with the results, but im assuming that in the next year if i dont go new mobo/cpu/RAM/GPU or SLI, i wont game everything 1900x1080p very high.
Hope this answers your question.