You haven't missed much... The nm reduction is just a smaller architecture... The smaller it is, the cooler it runs, and the less power it takes... which all in turn cause a faster machine. The 8800 if I'm not mistaken is known for it's large caches. Don't let the Clock speeds distract you... there are a lot more to them than just that.
If you're a big OCer... than smaller nm is always better. Also, smaller nm usually means it's cheaper to mass produce too. So it's better and cheaper, that's exactly what you want!