http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01080282&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&product=3436818&lang=en#N1203
Theres the motherboard information, looks good as the system is rated to take a processor which uses more power than what your system uses, up to 89W
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01048277&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&product=3436818&lang=en
That tells me your processor is rated as up to 65W
So the overall system including processor ought to use less than 100W, don't worry about the power supply inefficiencies. I feel good that you can use either:
- HD 5670 (any card, though go with the one William Watts linked as its dual slot and $85 after MIR)
- or anything weaker than that such as the 5650 or 5570 depending on your budget.
You could use something like a 5750, but at some risk to your computer given its age. I feel confident in a 5670 however.