IChat and "Insufficient bandwidth" for video chats

Question

I have spent hours and hours trying to solve this problem, after having spent a lot of money for an iSight specifically for use with iChat, which I now find don't work together. I just made the switch from PC to MAC and have been nothing but amazed until this... Here I have to give a huge "thumbs down", though. Apple has really screwed this one up.
The error when trying to connect video is "Insufficient bandwidth to maintain..." and sometimes "A communications error has occurred".
If anybody out there, especially APPLE can help me fix this, I'd be very appreciative. Note that I had GREAT success with aMSN, Yahoo Messenger, and Skype (for the mic).
Based on lots of reading, here are things I've done without success. Note that I have a very good ADSL-2 20MB connection! The machine is a new Mac mini Intel core duo.
1. Set the QuickTime streaming bandwidth to below what I know the ADSL line can do.
2. Set port forwarding for all the ports I found in all the FAQs, Apple technical documents, etc., etc.
3. Tried turning the UPNP of the ADSL modem both ON and OFF.
4. Made sure there is no firewall.
5. Tried every setting for the video bandwidth setting in iChat.
6. Tried both .mac and AIM accounts. (You never know...)
7. Made sure I have none of the known conflicting software indicated by Apple.
8. Made sure that Apple Remote connections are disabled.
9. Upgraded to 10.4.7.
I have actually read that some people got around this problem by connecting their MAC directly to their ADSL modem and (presumably) turning off its routing functions. But what am supposed to do with my 4 other computers, then?
Apple, please FIX THIS! There are abosolutely tons of people with the same problem and no solution.
Mac mini intel duo   Mac OS X (10.4.6)  

Answer 1

Ok Ralph... Thanks again.
When you said "two lots", I thought you may have meant that the DHCP server might be giving out IP addresses on two sub-nets (192.168.0.xxx, 192.168.1.xxx, etc.).
Since all of my machines have a static IP (except my business laptop, which I have to drag around to customer sites), I actually tried turning off the DHCP server to see if it helped. It didn't.
Also, I don't quite follow what you're saying about the NIC actually having two addresses (masquerading)... one from the modem and one from the DHCP. This should not be true, as it is the specific purpose of NAT to avoid this. Only the modem/router should really know "who is who" and relay packets appropriately.
The only pseudo-exception I can see for this is if the iChat software on the client and/or the AIM server is passing through the external/internal IP addresses as part of its data packets... ie., not in the IP headers to be handled normally by the NAT layer. If so, then this is just quite simply a design flaw in iChat... and a really stupid one.
One other point... Should I understand from your reply that if I set the modem up for "single user" mode (ie., turn off NAT and DHCP), then I would need to manually forward the right ports?
Mac mini intel duo Mac OS X (10.4.7)
Mac mini intel duo Mac OS X (10.4.6)