I've had the same issue with RH 5.2 running on Dell Hardware (PowerEdge). But, like you already found out on google, I (we, it was a team effort) did not find a solution for the problem. Once the card was selected (not always the same entry) the installer script worked perfectly. After the install the network card(s) problem never arose again, it only showed up during the PXE boot.
We never asked RH to look into this, it was seen as a minor inconvenience that could (was not always the case) show up during a fresh install. We did suspect the allocation of the network cards as the base of the problem.
Probably not what you wanted to read, but you are definitely not the only one that has this problem.