XP and Vista both ran fine on my MBP. Just be sure to get the 32-bit for either one. As far as ram is concerned XP/Vista 32-bit won't take advantage of all of it, but there isn't any support for 64-bit from Apple just yet so the keyboard backlight, screen dimming, "fn" keys, iSight, etc won't work. These features aren't missed too much except for the screen dimming. It's a real eyesore at night when it's at full brightness and you can't do anything about it.
You won't even notice the memory consumption so don't worry about it. I have 2GB of ram and eveything runs great. DO concern yourself with hard drive space. My MBP has a 120GB hard drive. I gave Vista Ultimate the privilege of 60GB and it hogged 20GB after all the updates were installed for itself. XP will suck up far less.