in this case debian AMD64 would be your choice. I am actually running it on a dell workstation with Intel Xeon CPU @ 2.50GHz
64bit releases are on debian official branch since debian etch (
http://www.debian.org/ports/amd64/)
As someone pointed you don't need an RT kernel if DON'Tt want to record 16tr+monitoring+fx, 48Khz and 2.67ms latency :-)
Moreover, bear in mind that a lightweight desktop environment is what you want for your system unless you have tons of ram and cpu cycles to spend :-)
So, whatever your choice will be, you should avoid fatter ones such kde or gnome and try instead awesome, fluxbox, xfce.