Bella,
Have a read of
this thread regarding server sizing. You'll be surprised what a small server can handle. Most people don't utilise their hardware anywhere near full potential. It's bought because we're told we need to have x amount of ram, x speed CPU etc etc but those ratings are put together based on usage peaks. I also think it would be fair to say that the figures are inflated to benefit "the machine" and keep hardware sales ticking over.
It's also important to note the difference between the various types of diskless workstations.
The one we're talking about here is a true diskless workstation in terms of everything that happens on it is actually running on the server. Web browsing, email, etc.. Basic system use can be easily done this way and nothing is local to the machine you are working on and the local machines needs next to no hardware or grunt to be able to provide that function. That's why it is so well suited to schools and business applications.
For PVR use this method likely won't give you want you want. Instead you could set up a PC (with the right display hardware and CPU speed), to network boot a mythtv front end from the server. In this instance the software is still being booted from the server, but the application is running on the client machine, using it's CPU, Video and Soundcard in order to perform playback operations. Network communication and the way Myth is built still means that the media, tv tuners etc are on the backend server but in this model the client is not a "dumb terminal".
The beauty is that the server can be the server for the dumb terminals and also be the myth backend (and any other server function you like).
Cheers,
Arkay.