the MFT on windows 2k/XP can be set to 12.5%[default]. 25, 37.5 or 50% iirc. i usually use 12.5%. this is taken up in an area called the MFT zone, if you run out of space the MFT zone actually shrinks to fit the data in, assuming that the actual MFT is small enough...
hate to sound like an advertiser, but perfect disk shows you both the MFT and the MFT zone and exactly where it is on your disk...