Getting greylisting to scale

Hi,

I ahve run greylisting at home for 9 months with great success (thanks 
guys!). Now I am looking to bring it into work, we handle 3 million 
emails a month so scaling is a whole new issues for me....we average 
around 2~3 emails per second with up to 20 per second during a spam 
attack, so I need to look at ways to reliably scale to that sort of 
input....

What areas will need work tweaking over the out of the box setup?

Looking for docs on this, URLs appreciated,

Specific Mysql tweaks?

anything else?

regards

Steven