On any fairly new Mac, the 2GB would usually be an indication of the RAM (Memory) size and has nothing to do with the size of the the Hard Drive.
The hard drive is likely to be anywhere from 80GB to 500GB but, alas, you have not told us what sort of Mac you have.
As for the HD size, stump up the cash and go for the 1TB, three times the size for an extra $30, go for it.
USB or Firewire, either connection will work well if you have a recent Mac, but Firewire is the better choice if your Mac is older.
If you are running OS 10.5.x, you have the choice of using the built-in backup software called Time Machine. It's completely automatic and easy to use.
Yes, you can put Mac and PC files on the same drive but bear in mind that the PC cannot read a Mac formatted disk(HFS+(Journaled) and the Mac cannot write to a PC formatted disk (NTFS) without additional software. These HD's come formatted so that both machines can use them but the format used is notorious for making files just go away!.
If you would like to provide more details about your Mac, we can look at options from there
P
I need to get some sort of back up, and I'm confused. I've got it narrowed down to a Western Digital Passport, but can't decide on the size. I have the small 2GB Mac, and I don't really need very much space right now, but am looking at the future.
So, my question is, should I go with the smaller, 320GB one for $90, or go whole hog and get the new 1TB(TB? who knew)for $120? Are they actually like a interior HD where you can go in and delete files and add new ones or do you just dump stuff in there and have no ability to maneuver things?
And, while I'm at it, can I put Mac files and PC files on the same one, or should I get two smaller ones, one for each.
All of this, keeping in mind that I'm broke.
Thanks

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