Well the latest Storagereview testbed (version 4) has just been put up, and that says that the
Hitachi Deskstar 7K400 is the best desktop drive atm, and that has a 8mb cache.
I would be inclined to think that the extra cache can make a difference in some situations, but doesnt give all that much of a boost generally, it needs to be combined with good firmware, platter density and spindle latency. I would guess diminishing marginal returns starts to kick in more heavily after 8mb so increased cache wont provide too much improvement.
I could be wrong though, im just guessing.
G