If you're planning to use Boot Camp, Windows will have access to all the RAM installed in your computer. Since no Intel-based Mac has ever shipped with less than 512 MB of RAM, that should be adequate for most users.
However, if you'll be using Windows in a virtualization environment, you must have enough RAM for Mac OS X as well as Windows, since both will be running at the same time. Microsoft recommends these minimum amounts of RAM:
For Windows 7, 1 GB for the 32-bit version or 2 GB for the 64-bit version, and I feel those are reasonable amounts—although more is certainly better.
For Windows Vista, 512 MB for Windows Vista, but Microsoft suggests (as I do) 1 GB as a more comfortable amount.
For Windows XP, 128 MB, but it runs better and faster ...