It depends on the processors. GHz are not all that matters. If you are comparing two identical processors where the only difference is the clock speed, then 3.2 GHz will provide approx. 20% performance gain.
GHz is the clock rate of the processor, in otherwords, the number of cycles per second. The processor will execute commands every cycle so obviously, the more cycles per second, the more often commands will be executed every second.
Without knowing the price of the 2.66GHz processor, we really can't say if $490 is worth it or not. Nor can we say if it is worth it for you because we do not know what you will be doing with the computer.
That's $1591 for a machine that has a much bigger monitor capable of HD resolution, includes Blue Ray player, and a GFX card capable of playing the newest DX11 games at HD resolutions, a larger HD, much faster memory and can be easily overclocked to 4.0 Ghz. That's $200 cheaper than the Dell w/ a 3.2 Ghz which basically would have trouble playing any game on a 20" monitor. If ya not into gaming, grab a 4100 card and sit with the rest.