The price looks great! I am not certain how the performance would be but Alex did a write up on that unit that if you have not taken the time to read may be worth looking into. http://www.mediasmartserver.net/2010/06 ... ata-vault/
There are many ways to migrate your data. Last year when I upgraded my primary server from an EX470 to an HP ProLiant I simply copied the data over time and then moved drives as you are planning to to. I also let the EX470 continue to perform my client backups.
I even upgraded to the 3.0 software just to see what it could do and offer me but it was not long before I decided that the 3.0 had little to offer me because my primary server was handling all my needs quite well. I continue to run EX470's for various tasks and testing both onsite and offsite because I still believe they are wonderful machines.
Its not my intention to talk you out of the x310 and I am not certain how long the rebate offer will be in effect but I would urge you to also consider the possibility of building your own server. Just putting that on the table will help you determine if the x310 is the right choice for you and so if you do purchase the x310 you will feel much better about doing so.
I love WHS v1 as do many others yet the thought of many wanting to migrate over to WHS 2011 on there HP MediaSmart or HP DataVaults and doing it on a headless system poses some challenges but it can be done. The task becomes easier with a debug cable. The point here is if in the near future you decide to install WHS 2011 would the EX470 or x310 be the easier path or going with a unit you built? If your even thinking of WHS 2011 then you should factor that into your decision making process or you could look at the HP ProLiant Microserver which I believe does have a VGA Port.
The one thing about the x310 is I believe the 3.0 software on it is a scaled down version with the media functions removed. This will not prevent you from streaming your media as you have HTPC's to do the heavy lifting there but if you desire to stream your content online I am uncertain of how well the x310 will perform. I did this with the EX470 and for the most part was able to do it but demigrator kicked in and would cause intermittant issues with the stream.
What ever you decide to do I wish you best. If you do go with the x310 I would be very interested in hearing how it works out for you. I do see the value in the x310 in a file sharing capacity and wanted my employer to place on at work to backup their computers. I got sick of waiting for them to realize how important this is so I decided to show them and placed on of my EX470's there which is doing a great job and has come in handy a few times.