Here's a box calculator that you can download (the demo version is usually adequate):
http://www.ggimages.com/spsim/index.htmlAll you need to input is Fs, Qts, Vas, and Qes.
I'm suspicious of the results the vent calculator gives you for multiple vents, but otherwise I think it works well.
Duke
edit 12-27-10: That link no longer works; I've been unable to re-locate this speaker simulator on the 'net.