I'd stick with the 64bit amd release of ubuntu. The ubuntu operating system with the 64amd release is built to take advantage of 64bit processing which is faster than 32bit. Plus the athlon 64 can do 64bit and 32bit processing at the same time. Meaning you should be able to run 32bit programs inside of 64bit ubuntu. Presumably the 64bit ubuntu release will take advantage of the 64bit, but other programs you use will probably be 32bit obviously.
On another note, ubuntu sucks, it's not really easy to use. I know something's up when installing graphics drivers for ubuntu was a pain in the ass along with learning how to configure fstab to make your hard drives work after install(i messed around with hoary hedgehog and the warty release).
Something much better is mepis linux. That distribution is a livecd which i consider much better than knoppix, easier to use, and can be installed on the computer and comes with absolutely all the software you could need. Also mepis takes care of the graphics card issue without any other pain, same with the hard drive pain i had with ubuntu. Mepis is also built around debian.
www.mepis.org Ubuntu really is a good linux os, but the installer on it could be a lot better, it's autodetection of drives still leaves you needing to configure fstab, and it doesn't take care of installing the drivers for your graphics card(but it will install the sound card drivers and nic...etc.). Not to mention the last time i've seen the live cd for ubuntu...you can't install that to the hard drive...there's almost no point in downloading the ubuntu livecd.
But yeah, check out mepis, it's freaking amazing, and you can try it out since it's a livecd, and if you like it, you can install to the hard drive and get dual booting with grub from it of course.
Also, is there a working binary of fs2 for linux, something along the lines of similarity with windows where you put the launcher and the cvs or whatever build of fs2 in the fs2 root directory(that's the small reason i can't play fs2 in linux)?