There's also a limited amount of memory-level parallelism available... with 4-DIMM sockets you might need an 8-socket machine to get a 32x improvement on large (memory-bound) sequential scans, which I'd guess you can get on top-end Power machines.

(You can probably get more memory level parallelism with random access, but your overall bandwidth will likely be lower... fully exploiting memory bandwidth is complicated and difficult to do for real applications).