I'm currently chatting with an HP rep and they are assuring me that the generic brand 300w PSU is sufficient for an ATI x850 Pro. I have a really hard time beleiving this. The PSU calculator said I needed 287 watts. So.... I feel at least a 380w name brand PSU is needed to be safe.
Edit: After beating around the bush for a while, trying to sell me HP power supplies, they finally told me that a standard ATX power supply will fit.
Edit 2: I already knew the PSU had a 24 pin connector, so I asked how many pins it had to make sure the technician knew what he was talking about. He told me 20. So I don't really trust what he said anymore.