I got my Sapphire HD 7950 off ebay for less than $100. Plug and play all the way! One of the best cards for a hack I think.
Hi Fl0r!anMy R9 280 is working fine both with Inject / Ati = True & FBName=Hamachi as well as Inject / Ati = False.
I had a old 5850 in mine and picked up a Saphire R9 280x BNIB for AU$150. Plugged it in and it worked immediately with no changes. Using it for gaming and it's great. I'd highly recommend a 2nd hand 7970 or R9 280x as the best hassle free bang for buck until either the NVidia 10 series or AMD Polaris cards are working OOB.
You on ElCap? Sierra? Other?
You shouldn't need to do anything special, just plug them in and boot (no need for Crossfire connection, doesn't work in OS X anyway).