Apple will add £137 to your bill if you want a 320GB hard drive put inside the new . Thing is, you can buy a brilliant 320GB hard drive for as little as £50 or so, and keep the standard 160GB disk that came with your new laptop as well.
It's a ridiculous fee. So when I bought my new MacBook, I just paid for the standard model and put a 320GB disk in myself. It's incredibly easy, takes a couple of minutes to install, and unlike many previous Apple models, won't invalidate your guarantee at all -- it's classed as a user-serviceable part.
I used a Seagate Momentus 7200.3 320GB hard disk, and not only can you pick one up for about £60 (still less than half what Apple charges for a drive of the same capacity), it's a much faster drive as well, at 7200rpm, as opposed to Apple's 5400rpm disks.
The drive you need is a 2.5-inch SATA, and they're available with up to 500GB of storage capacity. Seagate will sell you a 500GB drive for around £92.
To install it, simply back up your Mac with Time Machine (if you want to keep your current OS configuration, installed programs and personal files). Then take off the battery/HDD cover underneath the unibody MacBook, uncouple the old drive and remove the four screws around the edge of the drive with a small Torx screwdriver, clip in the new drive, and replace the security bracket and the battery cover.
Then stick your OS X DVD in the DVD drive and either reinstall OS X or choose to restore from a backup in the Time Machine menu in the menu bar.
And that's it! You've got a much higher capacity and faster drive, and it's cost you less than half what Apple would charge you. Plus, you've got a spare 160GB drive for future use as well.
Don't say I don't do anything for you.