Yea, Fast internet does not always equal good gaming, its alot more complicated then that. But your right about it being a mixture of difference things.
As for your issue, you don't want it working as just a bridge, since the modem uses PPPoA and the router can only do PPPoA. So it needs to do the authentication, but just have the DHCP server turned off. Also, the DHCP range of the Dlink needs to be above the IP address the modem is giving the router. But if you already brought the router back, then I won't go into it further. Gaming and networking over DSL is not for the faint of heart, since its alot more complicated. Plus even though there is less overhead with ATM, the process inherent in the standard tends to create a lag spike between your ISP and the ISP or ISP's of the people your gaming with. In short, you end up being "that guy" that lags the entire match. This isn't always true but...yea..ouch. Where do you live? I can ask around, I know a couple friends that work at different ISP's, I can see if anyone is moving to your area. If your more comfortable, you can PM me. Don't put anything more then the general town or county your from though.