When I last checked, IBM sold these x86 servers (I can't recall what the model line was called) that you could stack and made to behave as a single NUMA box. These compared very favorably to POWER servers in the same price range (32 cores on x86 vs. 6 cores on POWER7).

I don't remember the maximum scalability of these, but I doubt they would reach the biggest POWER machines. Problem is: if you need the biggest POWER machines, you're probably running something that isn't even portable to x86, so the comparison becomes academic.