You could take a look at this thread on XMBC on Synology hardware.
Yes you can install a GPU half height. Nvidia seem to have the best comparability for nix systems from what I have read. Here is some info on cards that are known to work in the N40L.
The only thing is that you may find the standard microserver to be a little bit noisy when running close to the TV?
I already have it close to the tv, doesnt bother me at all, already have a half height gpu too, and old nvidia g210... But the link you provided is running xbmc back-end on the synology... i wanna run the whole thing there wich i guess is a bit more complicated...