Yum-Arch is, I think, a red herring. Yum-Arch, according to pbone:
"This package only provides the old yum-arch command from yum-2.2.2 It should be used to generate repository informations for Fedora Core < 3 and RedHat Enterprise Linux < 4."
So that's not even relevent to CentOS 4 or 5 in any way.
Hadi: Have you tried downloading and installing the yum package from the CentOS mirrors?