I followed Eric's instructions and I now get very smooth updates on the 
desktop machine running RViz.  In fact, I can even run the desktop over 
802.11g (it's actually a laptop) and I still get nice smooth updates 
from the robot.  So perhaps in my case, the bottleneck when using VNC or 
remote X11 was in just displaying the RViz graphics.

Thanks for everyone's help!

--patrick