Obviously, we're going to need a speedy Windows-based PC to drive this wall-sized gaming monster. A decent processor and good amount of RAM are necessary, and DVD playing software is a good idea as well, in case you want to watch some movies on the big screen.
Seeing as how you're reading Tom's Hardware, you probably already have a PC, so we're not including it in the under-$1,500 cost we've projected. For argument's sake, if you wanted a PC exclusively for your projector setup, you'd probably be able to get something appropriate for $500, minus the video card. You wouldn't need a monitor for it, as that's the projector's job.
You could certainly use headphones for virtual gaming, which keeps the costs down. Alternatively, a 5.1 PC sound system is really nice to have in the ultimate gaming room - with an actual audio system, you can share a big-screen movie experience with guests, should you feel so inclined. We used the Logitech Z-5450 in our ultimate gaming room: it's a THX certified 5.1 audio system with digital inputs and great sound.