Of the two cards you presented, the 4850 is significantly better. The problem is, your PSU will not handle either card. The best I would use with that PSU is a GT240. A Radeon 4670, while cheaper and likely to preform as good on your computer due to your CPU, uses a little more power. The Radeon 5670 also uses a bit more power than the GT240 and would be pushing it.
 

Yep. It doesn't need additional power other than what the pci-e slot can provide.

Upgrade your PSU if you plan on a 4850 though.