A couple PSUs with enough power for your system and a 4850 video card:
CORSAIR 400CX 400W or PC Power & Cooling Silencer 420W
The Corsair is slightly more powerful on +12V 30amps vs 27amps for the Silencer 420W. Both have a PCI-e power connector for the video card.
I'm in a similar upgrade situation. I tried the Corsair 400cx in a Dell Vostro 220 before and it wouldn't work. I think there was a grounding problem. Dell also said that the motherboard won't support PSUs higher than 300 Watts.
I would think that the 4670 would be the best option, but I still want to get some confirmation from someone who has tried it in their Dell because it seems to have parts that aren't upgrade friendly.