And as for less quality; It doesn't get worse. It just gets better. There are 2 theories as to why they overclock less;
1.) AMD gets better at speedbinning (making) the processors, so they can limit what it can do better. This has some credibility, but the next one is perhaps better.
2.) At the beginning, when Athlon 64 3200+s were in more demand than the other Athlon 64s, AMD would take 3500+s and 3800+s and underclock them, lock them at 2.0Ghz, and then sell them as 3200+s. Ofcourse, a native 2.4GHz processor will overclock more than a native 2.0GHz processor. But when a native 2.4GHz processor is sold as a 2.0Ghz processor, then it will overclock much farther than the native 2.0Ghz processors.