Another update, did some browsing and it seems that I'm not the only one who's used the HP 3500 Pro as a base. Can't really find specs on the PSU except that it's a DPS-300AB-72 A, but lot's of posts on various forums of people looking to upgrade their HP 3500 Pro with a new GPU and PSU. No failure stories and everyone seems to replace their PSU as the first order of business.
So, ordering that new PSU and case and hope for the best. Should be there on Monday. I'll let you know if the PC blows up and burns the house down (or you could follow the news in the Netherlands :biggrin: )
Just compare the pins before you force it in the oem motherboard...hopefully it is atx standard.