When it loads ZBrush sets its best memory options based on whats present in your system. though it is not officially reccomended I do raise my compact mem settings to 1024 but I never change max polys per mesh since dividing above whatever level my ceiling was set to has far too often resulted in acorrupted ZTool.
Killing any TSR programs and things sitting in the systray helps as well.
Hope that helps,
Scott