Hardware specs for new cluster

I'm putting together a new web cluster based on Debian LAMP servers and
Ultramonkey and I need something that's going to handle:
 
4 million sessions/day
25 million pageviews/day
40 million hits/day
250 GBytes/day
 
The index page is around 230KB and is around 50% of the daily traffic
(pageviews).
 
Is anyone running a cluster in this ballpark? I'd really like to know
what hardware you're using.