Hi Jeff,
I ran AFNI, FSL, and Freesurfer on 32-bit Linux for years with no problem, this release:
Red Hat Enterprise Linux AS release 3 (Taroon Update 9)
(Latest kernel = Linux 2.4.21-53.ELsmp #1 SMP Wed Nov 14 03:54:12 EST 2007 i686 i686 i386 GNU/Linux)
Now I am running all three on 64-bit Linux:
Red Hat Enterprise Linux WS release 4 (Nahant Update 6)
I think both Redhat and CentOS will both work. In my mind, the main consideration is ease of applying security patches and installing the OS in such a way that all the video drivers, etc., just work. We get Redhat preinstalled from Dell and pay Redhat $60/year for academic up2date/yum patches. If I had a minion to manage my systems that knew CentOS, I could save some money. To me, it is worth paying Dell/Redhat a bit more.
(And I'm sure other Linux versions, e.g., SUSE, might also work; just haven't tried them.)
Donna