[PsyMan]

As Razi already said, speed depends on the game and the plugins used.

The type of the CPU is also very important. Older Pentium 4 CPUs tend to perform quite bad even if they have a high clock rate. Newer Pentium 4 CPUs, Athlon XP, Athlon 64 and Pentium M CPUs perform way better most of the time.

For example,
Soul Calibur normally runs at 60 FPS
On a dual core Athlon 64 3200 (at 2GHz) it runs at 70+ FPS.
On an Athlon XP 3200 (at 2.2GHz) it gets around 60 to 65 FPS.
On an Pentium M 740 (at 1.73GHz) it runs at about 45 to 55 FPS.
On an "old" Pentium 4 (at 2.53GHz) it hardly reaches 40 FPS.
On a "new" Pentium 4 (at 2.4GHz) it should reach about 50 FPS (since most games perform better than on "old" Pentium 4 CPUs)

As you can see the results vary. Note that NullDC does not take advantage of dual core CPUs (yet).