Can they be used? If they are PCI Express, then yes, they can be used. Should they be used? If this machine will be used for gaming, only the last one (HD3850) is likely to be usable, and that very likely on low settings.
HEC is not known for quality PSUs, but even if yours is only good for 2/3 of its label, you should be able to run a GTX560 or HD6870 on it.
Thanks for your reply. It's been many years since I put a system together; it's amazing how rapidly changes come about. None of the cards I have fit any of the slots on the x58 Asus board, so I had to obtain a new PCI Express 2.0 x16 card, and, since I'm not into gaming, I suspect my system represents considerable overkill. Still, it's all a worthwhile learning experience.