pretty much all external hard drives can be formatted how you wish, and I would use FAT32 for mac osx - windows xp inter-operability.
I recommend Seagate, simply because I have both an external seagate and I replaced my macbook pro's internal with a seagate, but I'm sure others will recommend brands such as Western Digital. a bigger concern is, what size and speed do you want? do you want a "portable" external or one that uses its own power supply (and probably faster speed)?