i was going to buy an rm850, but then i found a used ax850 (the old, gold certified one) for 30$ less.
Witch one should i buy?
The AX series of PSU's by Corsair is still their top of the line product. It may be cheaper than the RM850 because it is non-modular. If that's not important to you, then you're better off getting the AX850
The AX. That's Corsair's top of the line PSU. Like Irshaadh said, I would only get the RM if I really needed the modularity.