I just ordered a Mele A2000, which is an Arm system similar to the Raspberry Pi. I think I jumped in too soon, the problem I've discovered since ordering is XMBC with hardware acceleration isn't supported, and it may never be supported. It has some hardware acceleration under Android. Cheap ARM might be there in a year, but I don't think it's quite ready yet. I may cancel that order.
Would something like an Lenovo Q180 be good? Intel Atom, 4Gb RAM, AMD Radeon HD 7450A 512MB. It's still a few hundred dollars, which is more than I really wanted to spend, given the PS3 works pretty well.