Getting inside the ZBOX device is easier than unscrewing a child-proof cap. There are a pair of thumbscrews that hold the bottom panel in place. Just remove the two screws and slide the panel off. It takes a bit of force to remove the panel, but with a little tug, it should snap right off.
The two main components you can upgrade or replace are the RAM and hard drive. It only comes with a single stick of RAM, a 2GB DDR3 SO-DIMM module clocked at 1066MHz. Cedar Trail supports up to 4GB of DDR3-800/1066MHz memory, and with RAM prices being what they are, it wouldn't be a bad idea to pick up a second stick for around $15 shipped, especially if you're running Windows 7 or Vista. The stick that comes pre-installed is built by Samsung (Part # M471B5773DH0-CH9) and is actually rated to run at 1333MHz with a CAS latency (CL) of 9. Unfortunately, Intel's Cedar Trail platform is gimped by low memory bandwidth, which isn't a big deal for a streaming HTPC, though it does put a damper on gaming performance as you'll see shortly.
Zotac also tapped into Samsung's resources for the hard drive, a 2.5-inch 320GB SATA 3Gbps mechanical HDD that spins at 5400 RPM. It's not a high performance drive, though it does offer plenty of storage space to a hold a collection of movies, TV shows, music files, and whatever else you might plop on your HTPC.
One thing to keep in mind is that the hard drive is completely empty, meaning it's a BYOOS (Bring Your Own Operating System) affair. And since the ZBOX ID80 Plus doesn't include an optical drive, you'll either have to install an OS from a USB stick or via an external ODD.