File system for external hard drive?

Potentially stupid question.

Buying a HD and external enclosure. I use Ubuntu/Win2k on one computer and WinXP on the other.

I should be formatting the HD as a FAT32 drive if I want it to work with everything, right?

if you don't use ext3 BE SURE YOU DISMOUNT BEFORE YOU SWITCH IT OFF otherwise you'll be fscking it.