Windows 7, just like Vista before it, will fill up any available RAM with parts of frequently used software when idle. Because free RAM is a wasted RAM. After all when the memory is needed it can be emptied in miliseconds.
Memory allocation crashes have nothing to do with how much memory you have and everything with buggy code.
All the misc software (antivirus, msn, the download manager, the crappy thing you installed Gods' know what for...) that stays in the background also consumes RAM, apart of the OS. I'm talking about personal experience having (now) a 100% clean system. I've even reached 2,3 GB's of used RAM on an extremely dirty system, and load times with Crysis 1 were hell (poor HD...).
Although I agree on the buggy code part. Bethesda could put some work in their Skyrim RAM garbage collector. Alhough a game that big...I don't know.