You are venturing into the exciting (and scary) space of VDI (virtual desktop infrastructure). I have a little bit of experience with thin clients and VirtualBox, but not much experience in a production setting, so take everything I say with a grain of salt.
Oracle provides robust VDI solutions, see
http://www.oracle.com/us/technologies/v ... index.html for some ideas. I type this now on a Sun Ray desktop connected to a Solaris Server which has 6 to 10 guests running at any point in time.
You ask about sizing so I will tell you my experience. For RAM plan on 2GB to 4GB per guest, plus the host OS, plus what's needed for the thin client software, plus any other services running on the host, plus any overhead you expect to use. The server I use has 60GB of RAM and that it probably more than needed, but it gives us breathing room and room to grow.
The host we use runs Solaris which has amazing virtual networking capabilities. Our server has two 1gb NICs bonded (aggregated) together to the switch fabric. All of the guest NICs are virtual NICs provisioned by Solaris. Solaris can provision a virtually unlimited number of VNICs each with QoS settings, bandwidth limits, VLAN tags, etc. I rarely see more than 15% utilization on the server physical NICs. The beauty of Solaris' approach is that almost any number of gigabit NICs can be aggregated together and then sliced and diced into any number of VNICs for the guests, giving almost unlimited network scalability.
For processor counts, it all depends on the guest usage. VB is very good about not using cycles when it is not necessary. For active users, plan for 2 cores per guest. Our server has four dual-core Opteron CPUs and plenty of headroom during regular processing, even though the primary use of the server is unrelated to hosting guests. In other words, this server hosts guests "on the side." My experience is that virtualization technology of the CPUs is more important than speed or core count. Get CPUs which have all of the virtualization features used by VB and you will have snappy guests.
Since this is evolving technology, the industry is still learning about the trouble spots. For remote desktops I have found that Sun Ray technology does very well on the LAN (I have no experience with WAN deployments) for most office tasks. Things start to fall apart when the users try to play video, use 3D, use USB devices, use the microphone, etc. All of those little "extras" are experimental with functionality being anywhere from non-existent to working well under certain circumstances.
I hope that helps. Please keep the community informed of your progress. If there is more information I can provide, please let me know.
Cheers,
Marty