Slightly off topic but are graphic cards not available for the mac mini means will your regular pci express card not fit into it.When I asked the mac dealer guy he quoted 37k for the mac mini with 60 gb and 7k for 128mb card
The graphics card which you get normally in the market will not work in Mac.
Mac mini is very small to fit your own graphics card. For 37k it comes with Intel GMA 950 which is good to run Mac OS X but not good to run Windows Vista in Boot camp
iMac comes with a Radeon X1600 which is again good enough to run Mac OS X but ATI is knows to have OpenGL Games performance problems & there drivers suck (nVidiot here
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Mac Pro comes with either a Useless GeForce 7300 GT or Uber High End Quadro cards, & Quadro cards are not meant for gaming. Again good enough to run Mac OS X but not for running Windows Vista in dual boot with gaming.
The only Desktop is Mac Pro while iMac & Mac mini are closed systems which cannot be upgraded other then the RAM manually. If Apple doesn't support upgrading then you have to hack which will break warranty.
In Mac Pro, you cannot just buy a graphics card available in the market & plug it in Mac Pro. It will not work as Mac graphics card come with EFI Compliant ROM chip which only works with Mac.
Bottom Line : Mac cannot be upgraded, unless you call upgrading RAM as the only upgrade possible.