For 99% of what I do, my 700 MHz Dell is plenty fast. The exception is simulation, either Spice or home-made stuff. For decent human feedback, you need to see the result of a change within a few seconds. I occasionally run into circuits that need 20 minutes to simulate one transient, so I'd like roughly 1000x what any Pentium can currently deliver. I'm simulating ion paths at 1 fs time steps, over a 500 ns flight, times 10,000 ions for a Monte Carlo thing, and it gets tedious. Things like Spicing oscillators can be equally bad.
John