For some reason i only put a 160GB hard drive in mine, lol, don't know why:)
How big a hard drive you need depends on how you're going to use it. If you are going to partition it for basic back-up, then you need at least 500 GB, maybe 1 TB if you're going to download a lot of things like movies, specially if in High Definition.
But I wouldn't partition your hard drive, there's not much point, it's much better just to have a separate external hard-drive so that you can be fully backed up if your main hard drive ever fails completely.
The difference in price is negligible for what you get, so I'd always go for the biggest hard-drive you can. Even though for some reason that's not what I did, lol.
Is that geeky enough? :)