I am Stumped!

How to register a unique name in an internal Dynamic DNS; the outside
DHCP address of a multihomed server?

I have an ISA Server 2006, it has a DHCP address from my ISP and a
fixed internal NAT address on the inside. The PDC is inside the NAT
and running WINS, DHCP and Dynamic DNS.

The ISA Server domain named FIREWALL has this config
-----------------------------------------------------------------------------
Nic-1 fixed IP 192.168.1.1 FIREWALL.home.domain.com on internal DNS
and WINS
Nic-2 DHCP IP x.y.z.a Ureliable????.Mammoth-ISP.com set to DHCP
from my ISP
(I want a Dynaic DNS name like DIRTY.home.domain.com for Nic-2)

How do i keep the proper config for the internal side so when i ping
FIREWALL i get 192.168.1.1 but i can also ping DIRTY and get (x.y.z.a)
the current outside DHCP address from my ISP?

How can a multihomed external ISP created DHCP Address be uniquely
registered on an internal DNS Server?

Thanks if anyone can , Please.