Nor is there likely to be one, until we can reliably prevent (or at the very least, predict) electron quantum leaps (not something you generally want to happen inside your CPU).
Actually, heat dissipation is the main bottleneck with an architecture like the P4, or even the new Core 2 Duos. The Pentium 4 easily scales up to 5ghz+ with suitable cooling.
Personally I'd go with an AMD. Unless you actually want 3 pounds of copper on you CPU in order to keep it cool enough to operate.
The tables have turned - Core 2 Duo is king now. The Athlon X2 chips are still solid pieces of engineering (unlike the ridiculous hack the Pentium D was), but Core 2 Duo is even better at performance per watt, absolute performance, performance per dollar (at least at the mid range/high end - the $100 Pentium D 805 is all Intel has at the low range, yuck).