Are you absolutely certain that your application will not run on a newer Linux release? RHEL 5 and 6 (both of which are absolutely supported) contain compatibility libraries that allow programs targeted for RH9 to run. If you have the source for the program, recompiling it to run on a newer release would be fairly straightforward, too.
If this program really won't run on anything newer as you claim, you probably have no choice but to use a video card compatible with a driver version that will work with RH9. However, at some point such hardware is going to be increasingly problematic to keep working, to say nothing of the bugs, security holes, and other risks engendered by running unsupported, 12 year old software. Sooner or later you will either need to (1) find a way to make your software work with a newer Linux distro or (2) find new software that does the same task. You can choose when to make that decision, but it will have to be made at some point.