When we ordered a Time Capsule, we chose the 500 GB model, since this was sufficient and also less expensive than the 1 TB. However we had in mind that as the price of large disks came down, we would one day change the disk for a larger one.
This day came early since the hard disk in our 500 MB was unacceptably noisy. We replaced the noisy Seagate 500 MB hard disk with a much more discrete Western Digital 1 TB Green Power disk.
To follow our installation procedure (somewhat easy) see this article:Changing a disk in the Time Capsule
[Update] Not all Time CApsules are noisy; Apple certainly uses several different manufacturers - they are not all Seagates.
