The only Citrix client for the 770 was debian arm compatible. Unfortunately, maemo left debian arm compatibility behind in OS 2005. OS 2006 and later will not run the Citrix client.
I used that client on OS 2005 and it worked with a bluetooth keyboard for input.
However, OS 2005 would be a pretty big downgrade on your 770, I wouldn't recommend doing it.
Smallest best citrix setup I can recommend would be a Dell Axim X51V. Citrix for Windows Mobile works pretty well, and the external keyboard is not necessary. I've used this for a few months without any serious problems. However, Dell isn't selling Axims anymore, so a good used is probably your only option.