Routing behind NAT server

Hi,

My internet connection go through Modem router which act as NAT on
192.168.1.1

I'm configuring an Windows Server 2000 to provides a network of
192.168.3.0 with subnet 255.255.255.0. I'm leasing out DHCP address
through the windows server to 192.168.3.2~253.

I've 2 nic, the external nic is allocated dynamicly to receive an ip of
192.168.1.x. The internal network nic providing DHCP request has a
fixed ip of 192.168.3.254

I'm using RIP V2 to exchange routes info with the modem router
192.168.1.1. My internal networks routing works fine, but client under
subnet of 192.168.3.x is unable to access internet.

TCP/IP Setting
--------------
Internal NIC - Gateway - 192.168.3.254(Windows Server itself)
External NIC - Gateway - DHCP (192.168.1.1 - Modem Router gateway)
thanks in advance.