said by HiVolt:ThinkDiff pretty much nailed it with all the right answers...
To add to the graphics department... Even the Mac Pro, even as outdated as is and its Radeon 5xxx graphics, does not support and has not supported multi-gpu configurations like CrossFire or SLI, so even a totally redesigned version which is expected sometime in the new year will likely not have that...
Besides, for a Mac Pro's base price you could build a killer gaming PC with multi-gpu configuration quite easily.
So if you're an avid PC gamer, Macs are not a great (and never were) a good choice, due to always being gimped in the graphics department.
Would a hackin-tosh work for someone looking to have the GPU power and the ability to work in OSX?