How badly do you need it? Do you do work where you really WAIT on your
computer to finish the work for you?
If that is the case, upgrade to a Core 2 Duo immediately, and get
whatever your money will buy you, but do not shell out for anything
faster than 2.66 GHz, performance/price diminishes after that point.
When Quadcores come down in price over the next 6 months, you may want
to upgrade again, as you will see approximately a 1.5x performance boost
for well threaded code, which is quite decent.
regards , Peter B. P.