I've just gone through exactly this scenario trying to get a Firewire/USB2 drive working between my Toshiba Satellite 5200 laptop and my G4 iBook. It was a pain but I got it working in the end.
To answer your question, I bought a Seagate 200Gb hard drive and a generic external case (Oxford 911 chipset) and it works just fine. To go a bit further and let you know the pain that you may encounter trying to get it running on both, here is what you'll have to consider. (I'm not a big techie so this was me learning on the fly from forums such as these and a fair bit of Yahoo and Goggling).
Putting the drive in the case was the easy part. Trying to get the thing formatted was a pain. If you want to read and write to the drive using no more software than the standard OS then you need to format the drive to be FAT32. XP won't let you create and format a single partition of 200Gb so you need to be sneaky.
OSX won't format the drive to FAT32 (well, I couldn't find the option) so you need to do it from windows. XP wo't let you do a partition larger than 32gb or thereabouts so you need to download a Windows 98 boot disk (
www.bootdisks.com or something like that I seem to remember) and also a newer version of fdisk that can handle large drives (another search should find that quick enough).
Using a boot disk on my laptop meant that I didn't have USB drivers loaded (and I didn't know how to find them) so I ended up having to take the drive out of the case and hook it up to a regular desktop PC. To do this though, you need to use a newish PC otherwise the drive won't be recognised above 135gb.
So, in short for a drive that can be used on either PC or Mac:
Download Win98 boot disk
Download new version of Fdisk and save on boot disk
Plug hard disk straight into PC that can recognise BIG HDs
Fdisk to create a single large partition
Format partition with FAT32
Remove HD and put in case
Plug into Mac/PC and give it a go.
Enjoy!
