Nice to see that infiniteinfo is finally getting a new server. RH 9 is ok. It does present some more problems for Oracle 9i than RH 8.
But if the setup is like before, no Oracle. You will be fine with RH 9. One thing that you should test if aol2 can still run on RH 9. Also the fact that these sites run on pg 7.1. Hence there was 2 pg version running 7.2 and 7.1 at the same time.
RHEL seems to be a real solid choice, if you have enough cash to spare. I am not doing the sys admin stuff lately, but I am using a RHEL AS prod server. Really nice. One thing that I don't like though is that the dev environment is not the same as the prod server. I would not like to run an aging desktop :).
Good luck, hope the new server serves infiniteinfo well.