The OP needs both an external storage solution (500GB of files doesn't fit on his 320GB internal drive) and a backup solution (precious files).
Considering storage requirements:
If only occasional access to the media is needed with a MBP, an external attached drive works fine. But for frequent access, with a portable system, it's like having a ball and chain. Assuming Internet access via wireless with the router under the OP's control, a NAS (Network Attached Storage) or a computer acting as a file server (an inexpensive PC with a large hard drive would do) turns the chain into a wireless link.
Considering backup:
TimeMachine is nice in it requires no thought after setup, however it's a PITA with a MBP unless the backup is on a NAS or server because TimeMachine really wants to be running all the time.
The external storage and the MBP need to be cloned to backup drives (at least two sets), which should be stored offsite to protect against theft, fire, or natural disaster.
Happiest Solution:
would be a NAS capable of hosting TimeMachine backups, plus three additional drives (kept offsite), two to clone the NAS and one to clone the MBP. Not inexpensive, but secure.