I'm just going to have to find a reciever with audio out and hook up a surround sound or 2.1, right?
If you won't be running audio to the projector, then you won't need audio out on the receiver, but almost all of them come with that.
What you want is something that can connect to your Xbox 360 and cable box, decode the multi-channel audio outputs, and play the sound. You can do that with a receiver and unpowered speakers, and you can do that with a number of "PC" speaker packages. I like the receiver option myself, but I wanted to mention the other in case you were interested.
Some receivers that come to mind are the Yamaha RX-V663, Onkyo TX-SR606, and the Pioneer VSX-1018. All of those can be had in the $400, and all of them will process audio over HDMI. At this point in time, the lower end models (like the Onkyo TX-SR506 that can be had for around $150 on sale) don't process audio over HDMI - they just act like HDMI switchers and pass the signal through to something else. Someone here might know of cheaper ones that will do it, and I'll be happy to be shown wrong. There might be a lower end Yamaha that will do it.
When I mention PC speakers, I'm thinking more of connecting the digital audio outputs from the Xbox 360 and the cable box. I don't now if they have optical connectors, digital coax, or both. There are PC surround sound systems that will accept those connectors those.
EDIT: I feel kind of stupid for not thinking of this earlier. If you are going to use the digital audio ouputs from the Xbox 360 and cable box, then you could easily go with the Onkyo TX-SR506. That is a nice receiver at a nice price. It just wouldn't work for you if you had to process audio over HDMI.