If you're getting a nas, decide how much storage you need, double that, then add a little more. Unless you're very meticulous on cleaning out your drives, you'll fill it much faster than you think.
Its also a huge pain in the ass to add more storage capacity to a nas, after the fact. If you're using any redundancy (and you should be) then adding capacity is not a simple plug and play affair and you risk your data in the upgrade.
