Agreed, right now the only choice is a Mac Pro and 4 cores is overkill (though the Xeon for all intents and purposes is the best gaming processor, basically a faster Conroe, excluding memory subsystems that is...) not to mention non ECC non buffered RAM would be cheaper and faster (for games at least), a good mac for games would have to be full size tower (same size as Mac Pro or maybe a tad smaller like the G4 towers but make it ATX and "high profile card" compatible), they could have a decent base system and be customizable like the Mac Pro, see for example:
Base System:
Core 2 Duo "Conroe" E6600 (upgradeable, as in, "buy from newegg and drop in" upgradeable)
1GB DDR2
GeForce 7600GT (quite good for midrange)
SLI\Crossfire chipset
250GB SATA HDD
500-1000 modular dual 12v rail PSU (for upgradeability)
Options:
E6400, E6700, X6800
2GB, 4GB
7300GS, X1300 Pro (you know for the non or casual gamers), 7900GT, X1900XT, 7900GTX, X1950XTX, 7900GTX SLI\X1950XT Crossfire
+all the other stuff like HDDs, optical, wireless, etc, etc
In my opinion that wouldn't be too expensive and it would sell like hotcakes, if you want a headless mac with more punch than mac mini but you are not a gamer (or a casual one) you can get the E6400, low end card, etc to keep costs down but still have the option to upgrade with a good PSU and upgradeable CPU & GPU, and the gamers can, depending on their wants and budget, get whatever they want (you have everything, a good midrange card as base, a good mid-high range card, alternative, high end, high end alternative, ultra high end)
Sorry I got excited