the k5000 is gonna be great. has the staying power and stability of a quadro series card, but packs the punch of a true high end gpu. 3x the amount of cuda cores over a gtx 580. prelim specs say it may only need one power cord also, so you can pop two in a existing mac tower without overdrawing the system. meaning really good power efficiency. gonna be expensive, but i think totally worthwhile for all next gen tower computing.
the issue with using gaming cards is the fact that there's never any official support for these cards in pro software. you can use them, but if there is any issue, the support team from the software company can't diagnose the problem. plus gaming cards go off the shelf much faster, and don't stick around like quadro cards do. I'm using a hacked 470 myself for resolve, since the q4000 isn't really cutting it anymore, but I really can't wait to get a new workhorse gpu!