You should get one single gtx 670. SLI has several problems on graphics cards that aren't top of the line, from microstuttering to simply not being supported by some games. You'll see much better minimum framerates with a single gtx 670 or 680 than 2 gtx 560 ti cards.
I would personally suggest going for MSI's 670 power edition or 680 lightning, both are excellent cards for further overclocking ventures and have cooling solutions that allow it to best asus' directcu II.
edit: and if you want to keep it in the 300-350 range your looking at the gtx 660 ti for single-monitor setups and the 7950 for multi-monitor setups. Or you can just get the 7950 if you have the extra $30 for the performance boost.
7950- $330...