Yeah, I'm using something similar, memory is no big deal, since non-minecraft/java-related resources only hog about 75 Mb of ram, so Minecraft has almost 2 Gb to use. I'm more worried about CPU usage. It's using ~60-100% CPU on a laptop (2x1.8 GHz), it's melting!
At my job we've seen about a sixfold increase in speed when moving a heavy duty computational code from java to C++, but it all depends on what the program does, of course, but I think Minecraft would be faster using C++ (or just plain C, hehe) with SDL+OpenGL for graphics. But I think CK is a better judge at that, since I know very little about game programming.