ugh, coolermaster 500 really only put's out 400w, so your limited there. A 6850 isn't going to be much upgrade so I'd recommend at least a 6870, but your probably going to need a new psu for that.
For resolution, right click your desktop, choose personalize or properties, then display settings. Make sure the slider is all the way to the right (high). Below the slider it will tell you what the monitors resolution is.
can i also ask, hope its not much bother, but since im very new to all this "upgrading pc" thing, iv looked on amazon, and there are many different "brands" of 7750, like sapphire, VTX, what do yuo recommend for me, as the prices are different and im confused by how they differ from each other