One of the issues that you'll find with the Apple TV, or at least the original version is that it was seriously lacking in storage space. 40GB just isn't enough to store all of your media. Fortunately, some clever hackers have managed to hack the Apple TV to allow an external USB drive to act as the primary storage drive.
The hack probably shouldn't be attempted by a novice, though if you're good at following step-by-step instructions, you should do just fine. One of the great things about this hack is that it does not require you to crack the case on your Apple TV.
Once you have completed the hack, you're left with an Apple TV with a much larger storage capacity. It currently only works on the 1.0 Apple TV software, so you'll likely need to set it back to its factory settings to make this work.
Apple TV finally hacked for USB storage expansion [via engadget]