That's not true with Linode, you can add additional disk, memory and transfer under the extra's menu once you've purchased a slot. They'll migrate you as required.
Neat. I did not know that. Do they post that information anywhere on the public, you-haven't-paid-for-this-yet site? I've been trying to figure out what I'll need for hosting for the last few months. Linode looked best, but I worried about having to get beefing systems just to get more HD.