I mud from a slower system with a smaler screen. Here are some suggestions that make the client work quite well for me. Please feel free to add any that work for you.
Turn off alpha in all windows.
Turn off background images.
Use Java 1.5, I recently tried 1.6 and it was like mudding in porridge.
Turn off the map window when it's not needed.
Use monospace fonts in all windows.
and before any smartass says it....
Buy a new computer