Get an HD5670. It'll be much better than the HD4670, and can handle games decently at 1680x1050, and medium-low on 1920x1080. You'll definitely see an improvement from your HD3650, and it'll function just fine on your PSU.
Dell is known for using good PSUs which are actually a little more powerful than what it is rate for on the sticker; I would say maybe an extra 25w - 35w on tap. HP is not known for under-rating their PSUs nor for using above average PSUs.