Yes you can. It will have to be reformatted into the mac format HFS+ for Time Machine to recognize it. Time Machine will prompt you to refomat it when you start the program and choose the drive.
I recently purchase a 320 GB Maxtor One Touch 4 mini and am using it with time machine to back up my new 2.4 Alu Macbook and it has worked great. I don't leave it plugged in all the time, but when I do plug it in Time Machine auctomatically starts and updates the backup.