Full fledged Ubuntu with Unity 3D will certainly eat more RAM, though I don't really know exactly how much is normal. I'm running 32-bit Lubuntu 12.04 on an Acer Aspire D257 and it uses 90 Mb of RAM without other processes going.
I think, most of the modern OSes are made for computers that have 2 GB of RAM or more, in my experience.
If you don't have that much RAM, I suggest you try Lubuntu, which is very lightweight. Or if you plan to buy RAM soon, you can use Unity 2D until then (which is usually more lightweight than full 3D Unity).
Since you already have Ubuntu installed, you can run it like a Lubuntu system by installing the
lubuntu-desktop package. Then log out (or reboot, but that's not necessary), click the Ubuntu logo icon on the login screen, select Lubuntu from the drop-down menu, and log in.