This come sup all the time, and the single MOST important factor is your hard drives. You say they had a RAID card. What kind? How many drives of what type were connected to it? The processors have plenty of oomph to do what we throw at it, unless you start talking about High Definition video, where the processor can start to be a factor, but only after the harddrives. If they want some sort of a performance increase, the drives are where you are going to get it.
Also, if you are getting a new machine, then it has tiger on it already, go ahead and get enough RAM for it and upgrade to 4.0 the shows are backards compatible with 3.3 from a DMX standpoint, and the additional features are there for them to use at their will once they get comfortable with it.