Dell mobos will not work with off-the-shelp PSUs. Plug it in and fire it u... and you fried your mobo. That's one reason Dell suxxx.
Try the card I have, GeForce4 MX 4000 (Oh, I can just hear the fanboys screaming now...). I can play HL2, as well as any other 3D game, but I don't. The card is cheap now (was once cutting edge), and it doesn't suck up a lot of juice (my PSU is 250W too).
For a system like yours, and with what you want to do with it, an older, less powerful card is the ticket.
You may also want to look at the 9600's from ATI. They ain't bad, they are cheap, and they don't require your own power plant.