Using an app called android system info, or any task manager really, you can sort the processes by RAM usage and see what is still clogging it up. There may well be a bunch of crap still there from your carrier as opposed to Samsung and Google. Or it might just be getting eaten up by apps that you've installed, in which case consider uninstalling anything you don't need. At the end of the day though, if you start running out of RAM then android will kill all of those non-critical tasks, so I wouldn't worry about it too much.
Another cool app for rooted phones is AdFree Android.