USB driven ie I don't want to plug another unit into the electricity supply.
BAD idea...USB power doesn't have the voltage to power a hard drive during heavy usage...it's always best to get an external drive that has it's own power source
Must be pre-formatted.
Must be Vista compatible.
almost all of them will be....VERY few basic external drives require their own drivers...they just run with the standard USB drivers in windows and then the OS will detect the hard drive
160gb seems to be adequate?
as long as you're not wanting to store more than 160gb on the drive
Firewire usb recommended confuses me.
Firewire (for the moment) is capable of MUCH faster transfer speeds when compared to USB...however all PC's have USB ports (all modern ones anyway) not all PCs have firewire ports...so if you don't have firewire or you plan on moving this drive around (like taking pictures to a friends house) i'd suggest using USB
Difference between hard drive 'enclosures' and external hard drives confuses me
an external hard drive is a device that contains a hard drive and can be connected to your computer....an "enclosure" is a device that allows you to put your own hard drive into the case and connect it to your computer....basically something listed as an external hard drive...has the drive inside it already...anything listed as an enclosure requires you to puchase a hard drive and stick it in the box (which is why enclosures are always cheaper)
Where do combo units come in to the equation?
you'll need to be more descriptive as to what you mean by a combo unit