My system is Ubuntu 12.04 64 bit + Unity and it uses ~480Mb with no other processes-apps open, just the core system. Is this the expected amount? It seems a bit steep to me but perhaps this is normal behavior.
Yes, ~480 MB is normal for a stock 64-bit 12.04 with Unity (I get similar).
Don't forget that what you call the "core" system is actually running a 3D desktop environment as well as lots of background services. Windows 7 SP1 x64 with Aero is in the same region.
This shouldnt be a problem on a system with 2gb or more of RAM If you want to lower RAM usage, consider switching to a lighter version such as Lubuntu.