first off do a search, you will find what you need to know. but basically you are probably already on 2.3.340 (check system info under phone settings)
download droid x bootstrap from the market, and download Rom Manager.
open bootstrap, tap bootstrap recovery, grant superuser permission, it will say "success" open up Rom manager, tap flash clockworkMod recovery, it will flash it....latest should be 2.5.0.7.
go back to bootstrap, tap reboot recovery, the phone will reboot into clockwork recovery. here you can and should make a backup of your stock rooted Droid X....this will come in handy when you need to do an update.
after that, download some ROMs (GummyJar 2.5 is great) reboot into recovery again, install .zip from SD card, choose the .zip you downloaded, and install, and enjoy.
DONT FORGET TO WIPE DATA/CACHE BEFORE CHANGING ROMS OR YOU WILL RUN INTO PROBLEMS.