By default LimeWire tells the Java virtual machine limits itself to 128 MB of heap size. It doesn't matter that you have 2 GB of RAM. Note that the JVM also uses some RAM for JIT-compiled methods, thread stacks, native library code, etc., so the JVM may use a little more than 128 MB total, but the heap will be limited to 128 MB.

Which platform are you running? The way to change the parameters passed to the JVM is platform specific.

Which OS are you using?