There are some killer deals on the Seagate 320 and 500gb FreeAgent Go's, if you look around. Amazon has'em for $130 (500GB), and I saw a couple of specials at local Office Depots/Fry's for the 500GB for $99-119 a week or two ago.
That's the model I went with. I like it, but I wish it were as fast as my older Firewire drive (Alum. Macbook now). The one you buy that's not the "Mac version" will come windows formatted. You can just run Disk Utility on it, and partition it up however you like (Time Machine vs. Mac Normal vs. PC partition, etc) and then go to town with it. Personally, I made 3 partitions, just as I mentioned before, just to organize things better.