I have a 600W psu. It has the required 6-pin connector.
I have an MSI P55-GD65 motherboard and an i5-760(OC'd to 3.56GHz with a V6 Cooler). I can clock the CPU down again if needed(due to power limitations). But I'm planning on getting a new more powerfull 850W PSU sometime as soon as possible.
that board is fine! you can easily use that GT240 as a dedicated physX card (do you even play PhysX games?). you won't need to underclock.
is that a branded PSU? if so what is the make/model of the PSU if not then how much current (in Amps) have that PSU got on the +12V rails?