Should be decently quiet. That plywood should absorb sound nicely. My only concern is that wood is also a good insulator? Trapped heat = not good. The airflow looks like it will make things better, though. Like the idea of the passive vents behind the Mobo. All that wood won't distribute heat like metal will.
Looks good, though. Have fun!
Eric
PS. You're another one I need to teach to resize pics!
800x600ish usually works niceley.