Well AFAIK it is just like ultra fast RAM. More cache on Intels makes a sharp performance boost because the processor doesn't have to access the RAM as often when it has more cache. It makes less of a difference for AMDs because the HT design has reduced the latencies of accessing the RAM, as opposed to the performance penalties that accessing RAM through the Northbridge causes with the outdated FSB design, which Intel are still stuck on.
Im think that this is right but not sure. Someone more learned (maroon1, Apok) will probably correct me if im wrong.