I'm using a DWL-G510 with Mandriva, right now.
First, bring up a terminal session, and type >lspci to see if the card is physically installed correctly. If you see a line for "ethernet controller", then the card has been recognized. I can see that for mine, it uses an Atheros chipset. If that's the case for you, then you have two options: the first is to go to http://sourceforge.net/projects/madwifi/
and install their driver. The instructions you find there are very good, and installation, although it may appear daunting at first, is actually quite simple.
The second choice, and maybe the only choice if your board is not using an Atheros chip set, is to go to ndiswrapper at http://sourceforge.net/projects/ndiswrapper/
I'm using ndiswapper now, because it's provided by the latest Mandriva installation. To use it, you'll also need the drivers provided with the -G510 card on the installation disk.
I'm not familiar with rhel installs, so it's possible that ndiswrapper is provided with it. If not, you'll need to download either madwifi or ndiswrapper (and the instructions) from another machine, then port the drivers to the box you want to install them on.
There's plenty of help already given in these forums for both, so using the search function here will be useful.