PC Alchemy, a pretty decent media center store, has three Mini PC form-factor media centers up on their site now. Each of them cost $1199, and each of them are small enough to fit on top of whatever set-top-box you have now.
The M1B, M1S and M2B all have slightly different shapes, but all have a 1.82GHz Core 2 Duo T5600 inside, along with 1GB of RAM and a 100GB SATA drive. They also ship with a wireless keyboard and a remote, plus an HDTV Tuner can be added on for more dinero. It’s also Viiv certified, has a built-in DVD writer and gigabit ethernet. We’re buying eight.