About disabling application in ics 4.0.4 I don't know about this feature but does disabling application can help to reduce my devices RAM usage?
While basically agreeing with Dan, it depends on the app you disable. Some of the pre-installed bloatware runs services in the background, which are quite unlikely to be killed by the OOM (Out Of Memory) "manager". So disabling such an app, other apps can benefit from it: Memory no longer consumed by the disabled app is available to other processes, and their likelyness to be killed due to "memory shortness" is reduced.

Whether you really feel a difference is depending on how much RAM your device is equipped with: on devices with a small amount of memory it "counts more", with 2 GB and more RAM to command you might not notice (except for the battery effect Dan already described).