[edit] I've mastered the Google force, so I don't tend to use forums much for info, I am on the macrumours.com forum though.
Don't bother with universal binaries for games. Most games are written to run under Direct3D in Windows, and the conversion to OpenGL for Macs lowers their performance a fair bit. I'm hoping to get a 17" Macbook, which'll have XP for games.
Check out
www.tomshardware.com for their graphics card reviews, to see what kind of game's they're running to tax the most powerful computers available.[/edit]
Games you can run with all the graphics settings turned up full:
Doom 3, Unreal Tournament 2004, Half Life 2 and anything from the Call of Duty series for first person shooter mayhem. I can highly recommend the CoD series. It's got some extremely good missions, and I've played through all 3 titles twice in a row it's that good, and played a fair bit networked at lan parties, where nothing else got played after it came out. Doom 3 is just pretty graphics with no content in comparison.
Rise of Nations, Warhammer 40K:Dawn of War + Winter Assault add on, and C&C Generals + it's Zero Hour add on for real time strategy. Generals networked is an absolute hoot to play.
Neverwinter Nights + add ons and Star Wars: Knights of the Old Republic for some huge Role Play campaigns. I've completed SW:KotOR as light side and it took long enough. NWN is just massive though. I'm about half way through the second chapter of the original, and there are 2 more expansions to it too. Neverwinter nights looks great with a distance mod at high resolution, when you're slinging spells around.
Games you can run with the graphics settings at low medium-medium:
Battlefield 2 and F.E.A.R are the latest big name FPSs (BF is multi player, F.E.A.R. has an intense single player campaign and is kind of the benchmark for high level graphics and physics).
Elder Scrolls:Oblivion is so graphically intense that it cripples most machines, but yours should be OK on the lower settings.
Star Wars: Empire at War is a newish RTS that I haven't had time to look at yet, but it's the latest in the genre, and Lucas Arts make some really good games.
Other Games
The latest Need for Speed seems to be most peoples favourite racing game. I prefer the older Midnight Club 2 though, as it doesn't mess around with customising the car, and has a great track editor. Playing LAN we'd either come up with something we could rip round flat out, if every corner's done perfectly (messy pile-ups to dodge), or just do stunts/demolition derby.
Psychonauts is a good platform game, and, as far as I'm concerned, is up there with the old classic Day of the Tentacle when it comes to sheer style.
If you don't mind something a little older, Freelancer still looks stunning for a Space Combat game.