Plesk might be a better fit. SWSoft gives the requirements for running Plesk
, and they say you can run 100 domains on 256 MB with a Pentium II. That sounds really spartan if you ask me, but it does indicate Plesk doesn't use much memory.
Just a couple of warnings. First, Plesk doesn't have cPanel/WHM's depth, so it should be ok for basic administration, but you won't have as much fine grained control. Second, there is a good chance your provider will have to migrate your account if you want to change control panels, so decide which you one you want before you start putting things on your server.