first off if it is being hosted from a linux box than you can make it fat32 and both linux and windows can access it remotely, you only need it to be fat32 if you are planning to have the drive moved to a windows box later.
second off there are 3 ways to do this:
1) as you mentioned ftp, both linux and windows can use an ftp server as a remote 'mapped drive' though in linux I do not beleave it is a 'mapped drive' bu rather it is called a remote or network mount.
2) Samba, if you install samba on the box you can share the drive via the windows networking (aka samba) protocal, any windows box will see it as though it were itself a windows share. Linux can mount this type of share as well.
3) NFS, nfs will noyl work on unix variants to my knowledge, so if you want to map the drive on a windows box this option will not work alone, however even if you do samba for the windows boxes doing nfs for your linux boxes is a good idea.
setting up samba and nfs are beyond what I will type in this forum, configuring it depends on your distro, but good all-around docs can be found at the samba website, and at
http://www.tldp.org
