I think you have the right idea. A firewire/usb 2.0 drive would probably be it. What I would do is mount the drive on the Mac, open Disk Utility and format the said drive to the MS DOS format (aka FAT32), that way the drive can be read by both the PC and the Mac...now plug the drive to the PC and move the files. That's what I'd do, hope that helps
You could save money on the external drive/enclosure if both machines will be working at the same time. Just use a Cat5 patch cord between them and network away.