The PhysX PPU (see below) is a chip and SDK that is designed to perform complex physics calculations. It is being designed by Ageia. On July 20, 2005, Sony signed an agreement with Ageia to use the Novodex SDK in the upcoming PlayStation 3. This will allow developers to design games with complex physics without programming a physics engine, previously a time consuming and extremely difficult process. Ageia claims that the PhysX chip can perform physics calculations at one-hundredfold that of current CPUs and physics software.
Back in August, the 3700+ was $290. Now it's down to $234. By December it may even be down to $200 or so. The DFI LanParty UT nF4 Ultra-D motherboard just recently dropped a few dollars and is now at $125. There isn't any need to buy the SLI mobo unless you're going to use it, but with $600 you're not going to get a mobo, cpu, and 2x good video cards.
Today the eVGA Geforce 6800GT dropped a few dollars as well, hopefully it will keep that up until December and you can probably afford one of those.