I just did a total system overhaul and I purchased a Chieftec Aluminum tower with window mod, and 420watt power supply from NewEgg.com for US$95. A little more expensive than your average steel case, but certainly a lot cheaper than the US$175+ prices of, say, a Lian Li case... and that's not factoring in the cost for a power supply.
I'm no physics major, but I can tell you using the same ThermoEngine Heatsink w/7K rpm Delta 60mm fan, my system runs about 20-25 degrees F cooler now than it did with my old In-Win Full Tower Steel case.
That may be in part to the newer Athlon processor (2700+ vs. older 1800+), but I'm certain a great portion of that is the dynamics of an aluminum chassis.
Happy Turkey Day in advance.