If you want a gaming PC, I wouldn't buy a Mac Pro. I'd either buy from a different company, or build one myself.
Now, if I was using it for work, none of these cards would be a problem. If I wanted to buy this computer for work AND gaming, then it's still OK. In order for the Mac Pro to be inadequate for gaming, you'd need to find someone on the market for a computer as powerful as a Mac Pro, that also happens to play games. Now, that person also needs to play the most high-end games and INSISTS on playing with completely full settings, since the graphics cards available for Macs are already decent enough to run 99.9% of games quite well, although not from a bleeding edge point of view, where an extra 8 fps is worth paying an extra $300 for a better video card.
I think the Mac Pro line is fine. Another 2 cores would be a better option to offer.