While you seem more advanced than my post is geared out, I think other will benefit from all of this. Try the gentoo handbook. For almost all users, it is very detailed and very useful.
Specifically, the configuring the bootloader part is all you need to know for dual booting that differs for installing on a drive with another OS preinstalled.
Now, as far as partions go... I would recommend the following:
Swap, mine is 2gig(I have a 160gig drive and 1 gig of dual channeled ram in that box.)
Root(/), mine is the "rest of the drive" aside from everything else. It is about 153Gigs, as I recall.
Windows - I would leave 5-10 gigs for Windows XP, honestly. This allows plenty of stuff and growth.
Boot (/boot)only needs to be like 64Mb.
File systems are up to you. I personally use XFS in Linux, but Fat would allow for windows XP to see and exchange data in tha partition. Windows XP should be NTFS, period. FAT32 is not a good format for Windows XP as a master partition. You REALLY want to avoid it if you can. Swap stays unfilesystemed... it is swap space, doesn't need a filesystem. Boot is up to you as well, for ease I would use ext2 or ext3(I am using ext3).