I am running this card now. For me, it's a relatively cheap and simple solution.
Flashing R9 280X for boot screens/PCI System Information
The HD 7950 isn't produced any more (neither Mac Edition nor normal PC version), but you'll still find used ones at ebay (around 150€ I guess).
Just get one with with 2 miniDisplayPorts, 1 HDMI and 2 DVI, this will be flashable for Mac.
Btw, before recommending a specific card to you it would be interesting to know what you're planning to do and why you want to upgrade. Gaming? Pro Apps with GPGPU? 4K? Many screens?