My answer was that he should look into a Pentium 4 with Hyper-Threading because its second logical processor often provides a better user experience in just this sort of scenario. Intel has made this argument in favor of Pentium 4s with HT for some time now, of course. But I can't recall making a purchase recommendation to an average user based primarily on, say, a faster bus or an SSE extension. Like I said: interesting.
In other news, I'm about to build myself a new PC. The old KT266A/Athlon XP 2100+ combo is getting a little long in the tooth. I have an absolute load of hardware around here I could potentially steal for use in my new box, but I can't decide myself what to use. That P4 Extreme Edition is just gonna sit on the shelf if I don't put it to use, right?