First a disclaimer, as nobody has done any serious measurement, what we can do is no more than an educated guess.
It seems that adding much more RAM (the fastest kind of RAM you can find) to all machines would help most. So that all the databases fit in RAM, and there's plenty space left for tcp/ip connection buffers, per-process PHP memory etc.
Current database design is very centralized, so single kickass machine would probably help more.
Well, I do agree that a single kickass machine would probably help the most, you really can't get something that kickass for $3000, especially in a rackmountable format. I think it will be much cheaper and more cost effective upgrading pliny, if possible. $3000 will just not buy you a dual Athlon 2800/4GB DDR/2x 72GB SCSI disk rackmountable machine.