Ubuntu Networking :: NAT A Certain Port To An IP On Network?

I want to do something that should be simple... NAT a certain port to an IP on my network. *rant about how much I've tried, googled and failed*Our building has assigned us an IP Range (say, 10.0.0.x). They've also provided me with 1 public IP, which they NAT to an IP in that range of my choice (say, 10.0.0.50). I want to NAT port 6500 on that IP, to a different IP's port 80 (10.0.0.51:80) (i.e. come in on 10.0.0.50:6500, forward all the packets to 10.0.0.51:80).I've reset my firewall in Webmin, so it looks like this :Packet filtering : Input (Default Drop)

Code:
AcceptIf input interface is not eth1
AcceptIf protocol is TCP and TCP flags ACK (of ACK) are set

[code]....