The PSU is holding you back a little bit, i'd say that upgrade your PSU to something in the 650-850W range.
Corsair HX/AX series
Seasonic X-Series ( or platinum series )
XFX PRO series
Then you can pretty much pick any card out there currently, depending on your budget.
650W XFX PRO and GTX760 for example would run about $90 + $260 = $350
which would about double your framerates.