time machine question...

this all depends on just how much you want to back up and the files you work with.

 

1) There is nothing stopping you from using the 250GB drive as a backup. However, you will not be able to backup more than whatever the formatted size of the drive is. You would have problems with running out of backup space if you keep your 500GB drive anywhere near half full.

2) Obviously a faster interface (eSATA > FW800 > FW400 > USB) would result in quicker backups and quicker access times when you were using Time Machine's front-end. As for if you really need the extra speed...I doubt it. You could probably get away with a Firewire 400 drive as a good backup if not USB, definitely no pressing reason to spend more money on a faster interface like eSATA or Firewire 800. I think your speed gains your be minimal if any.

3) Yes, you can partition a drive and use one partition rather than the entire drive. Keep in mind that a drive labeled as a "500GB drive" on the packaging does not actually hold 500gb of data once formatted.