We're falling into the trap of believing that 'higher clock frequency (Ghz) = better' here. It's not only the clock frequency, but how much gets done.
The Pentium 4 has high clock frequencies because the architecture is quite inefficient and it needs to run very fast to compensate. Unfortunately the chips got too hot and never ran as fast as Intel had intended them to. A 3.2Ghz Pentium 4 will be slower than a 2.6Ghz Athlon 64 (single core, not X2) because the Athlon 64 architecture does more per cycle and doesn't need to run so fast.
Intel's Core 2 Duo processors are much more efficient than the Pentium 4, so a 1.8Ghz Core 2 Duo is probably at least as fast as a 3.2Ghz Pentium 4, even if it was only using one core. If higher performance can be obtained at lower clock speeds like this, it means that the chips run cooler and use less power, so a lower clock speed is generally a good thing if it can give similar performance nevertheless.
This is why both Intel and AMD have now adopted model numbers rather than naming processors by their clock speed, because people get confused and assume a 1.8Ghz Core 2 Duo is going to be much slower than a 3.2Ghz Pentium 4, even though it's better.
The performance difference between 32- and 64-bits is very small except in very particular cases, and often not seen anyway as most people are running 32-bit programs, like Civ 4, on a 32-bit operating system. It's just that modern architectures are more efficient.