Configuration for a very high traffic DNS server

[Ramprasad]

Hi,
  We run antispam services using  Spamassassassin for our clients. We
get upto 20000 mails per hour per server. All servers are Centos Boxes
running bind 9.2 with Dual Xeon + 4GB Ram 

Most of the effective SA tests are DNS based , and a lot of lists are
served locally. The number of dns requests are more than 100-250 per
second ( I can see numbers using dnstop ) 

How do I use the caching-only DNS server optimally. 
Is it a good idea to have a central DNS server with all other servers
connecting to this machine. The DNS Cache hit will be very high .. but
so will be the number of open sockets. 

Is there any special High traffic bind readme. Somthing like using a
RAMDISK for DNS Cache etc

Thanks
Ram