Server :: IPTables Setup For Symmetric NAT

I am having difficulties setting up Symmetric NAT through iptables.

First things first:
"A symmetric NAT is one where all requests from the same internal IP address and port, to a specific destination IP address and port, are mapped to the same external IP address and port. If the same host sends a packet with the same source address and port, but to a different destination, a different mapping is used. Furthermore, only the external host that receives a packet can send a UDP packet back to the internal host."

Need:
I am working on a SIP application and SIP apps face a problem with NATed networks. STUN is a solution to such a problem and my SIP application has an embedded STUN client functionality.

Scenario and Technical Details:
192.168.0.200
+-----------------+
| ClientA - My IP |
+-----------------+
|
|GW:
| eth0 eth1 (example public IP address)
| 192.168.0.1 | 123.123.123.123
+-------------|-------------+
| NAT1 |
+-------------|-------------+
|
|
|
stun.1und1.de |
+---------------------------+
| STUN Server |
+---------------------------+

I am using WinSTUN, which requires a STUN Server address (such as the one I specified above) to return my type of NAT. What I need to achieve is Symmetric NAT through iptables, on the GW server, only on my IP address (192.168.0.200). I don't want it to affect the whole network. I am running CentOS release 5.4 (Final), and iptables v1.4.10