Chris,
BeamBoy is a quick and simple freeware program that will calculate the deflection of beams supported in various ways. I just used it to calculate the deflection of an aluminum bar 1/2" thick, 2" wide, and 12" long. With simple supports at each end, and a 10 lb load in the middle, the bar will deflect .00173 inches. You can download the program here.
http://www.geocities.com/richgetze/
It too me longer a lot longer to compose this reply, than it took to set up BeamBoy and perform the calculations. It defaults to using steel as the material, but for aluminum, all you have to do is change the Modulus of Elasticity under "Beam Properties" from 30 million psi to 10 million psi. If you try it and have problems figuring it out, let me know and I will walk you thru a quick example and you will be off and running. It sure beats guessing about material sizes.
Jeff