With the configuration given, I don't think there should be any problems with using IDE's like Netbeans or Eclipse. Are you using Windows as your primary OS? That might explain a lot of things. :-)
If your OS ends up eating a lot of memory by running a lot of background processes and applications, there is little point in expecting Eclipse or Netbeans to run blazing fast. Try the Windows forum of Daniweb in case the find that the problem is really the OS and not Eclipse/Netbeans. ;-)
As far as a lightweight IDE is concerned, try using jEdit.