Are you sure you want to make a 1 TB system from two 500 GB drives? That would mean a RAID 0 array, which is pretty risky--you will have absolutely no redundancy. If one of the two disks fails you lose everything. Definitely not for production use, but it's quite fast and great for test labs or environments with the proper methods in place for keeping data backed up elsewhere.
If there's nothing of value on the server at present then you could re-install RHEL5 on the box, using its graphical installer to configure your disks and create the RAID array at the time of installation.
Again: please make sure you understand the levels of RAID, and that you don't actually want a RAID 1 (also called a "mirror") array instead.