I have a similar dell computer, its a 7100 xps, and it has the same 460w psu. I just got my gtx 660 ti ($230 on newegg + $150 f2p games
and its working fine. I'm going to order a new psu for peace of mind, but it isnt that bad.
Also, how come you're going on the nvidia boat? I'm only on it for my video editing software (adobe cs6) which only supports gpu acceleration from nvidia. Otherwise, the 7870 and other amd cards are much better for your $.