Dual Core Benefits
WinRAR + DOOM 3
At stock speeds, Doom 3 frame rates dropped by 32.3% (45.7 fps), and at overclocked speeds, Doom 3 frame rates dropped 35.5% (63.1 fps).
So dual core did help here - we would expect a roughly 50% drop in frame rates without the second core.
WinRAR + LAME
Now this test is much more like it... At stock speeds, LAME was only six seconds slower in encoding; totally insignificant. Here we basically were able to run two apps at single app at a time speeds :) At overclock speeds, the difference dropped to two seconds.