I think it depends on your intended use for the PC. Gaming will generally dictate a more powerful card. There is a fantastic website called gpureview.com which lists lots of specifications for all types of video cards. It is very well designed, and has side by side drop down lists to let one compare two cards to each other.
It also has detailed info on the GPU chips in those video cards, as well as power usage generally required for the cards. Spend some time there, and you will almost be a PRO at video cards!!!
I think alot of this depends on if he is still agp or on PCIe for that pentium D and what he's playing.
If he's still on AGP then the best card, imo, is a 3850, maybe a 4760, but the interface bottlenecking becomes apparent in heavy textured gaming like a large Wow raid party.
If you're PCIe then get the best card you can and overclock the PentiumD. Its still a viable gaming cpu where in most cases you don't need raw computing power. For example: The PD 940 scores 144 in CoD2 and a Quad core E6600 only gets 181. http://www.tomshardware.com/charts/cpu-charts-2006/Call...