You most likely received this message during the boot process during the POST. The message means that the one of the DIMMs inserted is out of "Revision". It's a Dell thing letting you know that the RAM inserted is out of the board's spec. Doesn't mean that the RAM will not work. But you should look into the BIOS and make sure that your settings are correct. I have seen this (and other "Rev" error messages) and never did anything about them....they never came back to haunt me.
-Michael