<p>I want a Java IDE with a compiler for Linux and that does not require a a big amount of RAM.
I have 256 MB of ram, so my options are limited</p>

Does the 256MB of RAM also include program space, or will the code be stored elsewhere in flash or another memory type?
You can get embeddable java machines, and small java compilers for linux.  The IDE, however, is tricky as you don't merely say editor, but you want a whole integrated environment.
Since you have Java installed already, I suggest Eclipse.  It seems bulky and resource intensive on PCs, but you can cut it back significantly by removing features (classes) you don't want/need.
-Adam