HP P6230y - upgrade graphics, power supply?

I'm hoping I put this in the right place, if not I hope someone will give me a heads up!

I recently received an HP P6230y desktop from my parents, who replaced it with an iMac. Rather than build my own gaming PC, I've decided to see how well this thing holds up; it's got plenty of power and I am thinking it really only needs a boost in graphics. I was leaning towards one of the two; a Radeon 6950 2GB or a GTX 560 Ti 1GB. I've looked around on the forums and see a new power supply may be necessary; if so, it will be this Rosewill 450W. Good old Newegg.

My question is, which is a better "bang for your buck" graphics card, given that it's really the only upgrade I want to make? The price difference is minimal, so I wanted some expert advice. If there's a different card that may be better for what I need, feel free to suggest. I'm not sold on those, just what I came up with on my own. For what it's worth, I plan on playing mostly RPGs (Mass Effect trilogy, Diablo III, maybe SWTOR), and the lady may play The Sims. Who knows.

Thanks all in advance!