That won't be too difficult, most current cards support Shader Model 5.0 these days. Best fit for you would be a Radeon HD 5670, or the slightly faster HD 6670. Either card would be much faster than your current GPU, and would be able to run it on a Dell 375 Watt PSU. You may want to check for a BIOS flash from the Dell website to ensure PCI Express 2.1 compatiblity, some older boards have issues with the 2.1 cards.
If you need something that is PCI Express 2.0 only, something like the Radeon HD 4670 would also work with your PSU, and would still be faster than your current card, while getting Shader Model 3.0 support.
If you want cheapest, bottom of the barrel, something like a HD 4350 or 5450 would also get you Shader Model 3.0 support, but performance would be at best comparable to your current GPU, those two are not powerful cards, and aren't really geared towards gaming.