Those are generally do-it-yourself upgrades. They're really easy to do, by the way, and at least a processor upgrade should come with detailed instructions for how to install it.
The largest hard drive you can just plug and play without installing any extra drivers is 120 GB. The Cube's hard drive controller can only see 128 GB, and it can only be circumvented by a high-capacity driver. I've heard good things about the hi-cap driver, but you might not really need more than 120 GB anyway. Here's a good cheap one:
$81 shipped.
There are several Cube processor upgrades out there. Macsales.com has several available from PowerLogix, GigaDesigns, and Sonnet. The cheapest one, though, is
this Sonnet 1.2 GHz, which you can get from NewEgg for just a little over $300.
As I said, upgrading these components shouldn't be too hard. The Cube's innards just slide right out, and once they're out you can take out the old hard drive and put in a new one, and upgrade the processor too.