If there is a ICS ROM available for the Droid X, it should run on a device with 512MB. The minimum requirements for ICS specified by CyanogenMod team is 256MB and a "decent GPU". I am not sure what kind of GPU the Droid X has, but you will probably have a better experience with Gingerbread then ICS on that device if the GPU isn't that great. My brother had that device and complained constantly about lag and lock up. If you have the "stock" MOTOBLUR, you will probably have a better experience just by installing any AOSP based ROM.
You can expect the footprint to be about the same, maybe a bit less, then what you see now. This is because the android OS will always try to use as much memory as possible in order to try and keep your experience better. If the application is in memory you can switch back to it faster then if it has to reload the entire application, plus it can "remember where you were".
As a side note, I don't currently see any CyanogenMod 9 builds available on the CM forums for the Droid X, but that doesn't mean that there aren't any, I just don't see them.