Hi all, a question about buying an iMac at the Apple Store. I want to get the 20" 2.4 Ghz iMac, but I want it with the 500 GB hard drive, not the off-the-shelf 320 GB. Can I get this at the Apple Store? Do they stock iMacs with varying configs, or will they do my upgrade in some super secret lab in the back of the store for me? I know they do RAM on-site. Do I have to order it? And if so, can I order it at the store (and thus pay at the store with cash - I really don't want to put this on a credit card if at all possible).