Does the orginization that you work for own the real 64.xxx addresses? Or were they already provided for you by the service provider? If you own the addresses, then the advantage of using the real ip is that it won't change (unless you merger with another org and for some reason have to hand the real ip's back) If you don't own the address, and wind up changing providers later on, then using NAT has the advantage of not rec-configuring the DMZ devices during the provider change over.
Another area to look for is if you are using protocols in the DMZ that do not lend themselves well to NAT. If this is not your case then one other factor to examine is this: if you decide to use NAT, what is the level of difficulty that you will face during the IP address change (i.e., LDAP database reconfiguration, dhcp/dns, internal route changes).
As far as the pix firewall is concerned, it will operate at the basically the same level of efficiency whether or not you use nat - I would be surprised if the traffic takes one more millisecond to process due to NAT. I have not read doc anywhere that suggests that the PIX performs less due to nat/pat.
My opinion is to figure out what internal route and internal/external dns and config changes need to be done to accomodate a real-to-private ip address change and let that be your determining factor,. An this assumes that you own your address. If not, you may want to keep the real as is, but go thru the what-if-I change exericize to realize what needs done in the case of a provicer change.
Let me know if this helps.
