It depends what kind of game you want to play (ideas taken from Apple online store)
-Your options for first person shooter games are limited because of the MB's graphics processor, you could probably play Call of Duty 2 at very low settings and be ok. There's a few Youtube clips of folks playing Call of Duty 2 on Macbooks. If online multiplayer is your thing the Unreal Tournament 2004 will probably be ok.
-If you're into magic and fantasy type games and are interested in paying a monthly fee for World of Warcraft, your MB will run it but people differ in opinion on how well it plays due to graphics lag
-Real time strategy games, Age of Empires 2 or Rise of Nations would be good options. If you're not familiar with RTS games, it's a 'build a civilization/city, then build an army of different units, then use said army to destroy the computer's army/city.
-Universal binary games are meant to play with equal performance on Intel (current model) and PP Macs (older generation Macs like IBooks). Games that are not UB, like Rise of Nations, require the Mac to do an extra conversion step to play the game. The program it uses is called Rosetta. The only significance of this means that your Mac has to do a little extra work for the game to play and your gaming performance will suffer a little. Having said that, games like Rise of Nations are not demanding on a Macbook.
-There are some games you can download from Apple's 'Downloads' section, I have not felt the need to do this but it might be worth a look
