I'd go with a console, unless you're willing to buy a copy of windows to go along with it. The Mac's selection of games isn't very big and more resource intensive games are kinda lackluster in comparison if it's a native windows game.
If you go the console route, I think a Playstation 3 would actually make a better counterpart in my opinion since it has a Bluray player to make up for the one the Mac doesn't have and I do enjoy Valkyria Chronicles so far and appears to have interesting exclusives like Gran Turismo 5, Disgaea 5, Metal Gear Solid 4 and that Shadow of the Collosus/ICO combo remake. I haven't kept up as much with the 360 but I hear it has some good arcade style Shoot 'em Ups like Mushishimesama if you like that sorta thing. They're designed for use with a home theater system so connectivity may only require one or two
HDMI cables. Unfortunately they're already over halfway through their intended lifespan and probably have nowhere near as much wow as the latest computers might but at a retailing value of $300 bucks they're far more affordable than even the cheapest of new macs, giving you more to bank on in the future when it's time to upgrade to the next generation of gaming.
One of the bigger questions is if you'd really rather have a new Macintosh just for games, is if one of the new Macbook Pros would actually suit your purposes. The high end 15" is more portable which can be useful for LAN parties, still has a 1GB GPU and comes with the newest Quad Core Sandy Bridge processors. You'll also have the Thunderbolt port future proofing your investment for general use a little and more importantly. Since it's just freshly released, you'll get the most resale value out of it too.
Yes yes, I realize the iMac has a much bigger much more impressive screen. However if you're looking at HD consoles I'm assuming you have an HDTV, otherwise you wouldn't be able to utilize them to their fullest potential. If my assumption is correct, this means you could offset the problem by running the laptop in
Clamshell mode with a
A minidisplay port to HDMI adapter and if you have a surround sound system, utilize this
TOSlink to minitoslink cable for audio since I don't think the current Minidisplayport to HDMI adapters carry sound data.
The iMac does come with one of the highest resolution displays in a consumer product though and if you're willing to buy second hand, perhaps you can find a decent deal since the refresh cycle is nearing its end. It also has a higer potential for RAM upgrades with four memory slots too. Its nearing the end of its logical cycle though so keep in mind it's probably not quite worth full price, since apple doesn't allow big discounts on their products.
Okay, so the new macbook pros can support 16 Gigs of RAM, albeit the current cost is an arm, a leg, an empty stomach, a head and maybe your first born child. It means good things for the long haul though.
