I've recently found out that xserver can serve a desktop over the network. I.e. the user would connect to the xserver to use programs not installed on their pc.

I use bittorrent since it's cheaper/ less noisy to leave my nas on. So it would be useful if I could install something lightweight like fluxbox. However being a nas it doesn't have any graphics card. I', too cheap to buy a USB VGA.

Could xserver send the gui so I can see the gui on my pc?

It is quite possible to use X on a system that can show X clients and get them served from another. You can even do it using something like Hummingbird Exceed or Cygwin/X on MS-Windows to see the X from the server. You can even use a GUI console and access it with vncviewer (which uses X windows).

However, you say nas so I'm assuming this is a prebuilt appliance used for that purpose. If so it may not have or allow for the full set of Linux utilities that you'd expect. That is to say embedded Linux on appliances are typically stripped out versions that only allow you to do exactly what the appliance manufacturer built into it.

Actually I forgot to mention this would only be useful if I can initiate an x session then shutdown the client pc but still have that session active for later.
The idea is so that I can download stuff using my NAS without having my pc on at all. I have already flashed the nas with custom firmware that contains a full distro. At the mo it's running asterisk,samba and cups.

Why don't you just use a CLI/Curses torrent client?
Fire up screen on the NAS, get the torrent going, and
off you go ...


Cheers,
Tink

Sounds like you might be looking for something like xvfb.
http://en.wikipedia.org/wiki/Xvfb