Hi kumar, I'd say you need to get yum working first.. Blindly installing rpms is probably the worst way to go about upgrading anything in a Linux distribution.

I strongly advise you to contact redhat for support with RHEL5 and tell them yum does not work or install centos instead if you have not paid for/cannot afford support.

Then you can do as bubby recommends and :

su -
yum install gcc43
exit