Yeah, quite seriously, as much as you can afford. On my 15" I am getting 1.5GB of RAM, and although this isn't an option for the 12" (Only supports up to 1.25GB) I'm just trying to say, as much as you can afford. Keep in mind though, Apple overcharges on the RAM and your better off buying the RAM from elsewhere if you feel comfortable installing it.
Cheers,
lieb39