The Xms is the minimum value that Java is going to use upon start up, it will always be using at LEAST that much memory. The Xmx is the most it can ever take.
You can technically raise the Xms as high as you want (within the limits of a 32bit application), but you should be aware that at startup it will always be consuming that much. It's not necessarily a "bad" thing, just something to be aware of.