recently started using RubyMine 2.0 beta 3 on 64-bit Ubuntu Linux. With opened project, but without any file in editor the IDE uses about 200Mb of RAM. On windows it's about 70Mb. I'm using 64-bit Sun JDK.
When I started the application under IcedTea (default on Ubuntu) it looked terribly. After setting GTK theme, it got a bit better, however that wasn't still comparable with the Windows or MacOSX version. So I tried installing Sun JDK, which contains Nimbus, it looks OK now, however I would prefer native L&F.