You can partition the drive via Disk Utility. Say you buy a 500GB drive, since your sig says 120GB, I'm assuming this is the computer you're talking about. So you could set 200GB aside for Time Machine, and the rest for storage. PRetty easy to do - just connect the drive and stuff, open up disk utility, click the drive and click partition, then drag the divider, or type in what size you want each partition to be.
It will mount on your desktop as 2 separate drives and you can designate Time Machine to only back up to the one you want it to.