250w Dell PSU. About 65 watt limit for GPU.
Uhh you'd killing me here. Is it a low profile case or standard mini tower? If you want decent graphics you will want a new PSU and a more powerful card. With what you have I wouldn't do much more than an ATI HD5570. or GT 240. Both of which should fall under 65W.
The GT 240 will outperform the 5570 however the 5570 is DX11 and can also be low profile, if you need that. I'd probably go for the gt240 if you can, JUST MAKE SURE it is the DDR5 type not DDR3 cheapo.