It's not hard to compile a 64-bit client, there's a few threads here and there about it.
Say you wanted a brand-spankin'-new (kind of) FSM client, all you would have to do is:
sudo aptitude install build-essential subversion
sudo apt-get build-dep tremulous
cd ~
svn checkout http://fsm-trem.googlecode.com/svn/trunk/ fsm-trem-read-only
cd fsm-trem-read-only
make
sudo cp ~/fsm-trem-read-only/build/release-linux-x86_64/tremulous.x86_64 /usr/local/games/tremulous/
This assumes that you installed from the tremulous.net binary.
If you installed through Synaptic, I'm afraid that I've forgotten the location to place the client.
Read up on compiling, it's awful useful.
I might've forgotten something, someone please inform me if I missed a step.