Those Small Form Factor enclosures may not be the best thing for running a server but it comes down to how much data you need to store.

If you have an external DVD Drive then you do not need to have one installed in the enclosure and getting rid of it to provide a hard drive spot will make a big difference.

Check the motherboard out it may only have 2 SATA Ports and if so your limited to hard drives and the motherboard maybe an mITX or mDTX. A mITX will only have 1 expansion slot where an mDTX will have 2 expansion slots. Not sure if they are populated or not.

Check the motherboard specs to see if the NIC is 10/100 or 10/100/1000 if its only 10/100 you will not want to use this as it will slow things down on your network if your running Gigabit Devices but if everything on your network is 10/100 then it will not matter.

As for the Processor well that will come down to what the BIOS and Motherboard will support and then how much wattage can the motherboard support as well. The 3800+ is a dual core processor but you may want a Quad Core Processor to get good performance out of the server. You will need to check with HP to see what processors can be used with that computer and take it from there. Also check how much RAM can be used because you will really want 4GB of RAM to run WHS2011 well of which it will use like 2.2GB or ram when operating.

As for the system drive in WHS2011 I would not use a 2TB Drive, I would install a smaller drive for the OS and use the 2TB exclusively for data. Also WHS2011 has an option to create backups of the system drive so you should have a dedicated drive for that since its the backup drive or an external backup image of the OS drive that WHS2011 uses to perform a server recovery with otherwise you will need to perform a new OS install which makes for a really bad day. You could install 2 drives in RAID 1 Mirror for the System Drive something I have done in an EX485 but I used an ICY DOCK device which takes up one SATA Port and hard drive bay but holds 2 2.5" Drives be it Mechanical Drives or SSD. I also for additional redundancy did setup a dedicated drive to OS Backups.

You could consider expanding storage capacity outside the box. If its USB2 you will not get the speed you might desire out of the drives if its eSATA and it can support a port multiplier you might want to go that way but for me the best way to go is to have all the drives inside the enclosure when possible and only expand outside the enclosure only if you have to but do it as a last resort.

Last thing to look at is the PSU which HP installed to see if its proprietary or not. If you can add additional drives you will want to make sure the PSU can support the added drives and in a small form factor unit the PSU can be very small 150-250W. Thing is if you start adding things which put a demand on the PSU it may not be big enough to support those additional devices. If its proprietary then you may not want to even deal with the unit and its as standard PSU then you may want to install something with more wattage depending on what you do with the unit.

One thing to think about is that if your WHS2011 is as I believe an OEM version then your license is only good for the equipment you install it on and if you opt to install it on another system it may or may not install so for this reason you will have to be really committed to the equipment you install WHS2011 on.

Review what I wrote and if you have any additional questions or concerns let me know and I will be more then happy to help you out.

If you have it collecting dust, better put it into some use. You can certainly use it to test run the new WS2012E.