i m thinking about it. it is very easy to change which connection you upload to. that is just your default gateway. on the other hand, it is difficult to control where data come from (nor would you want to). the routers of the internet decide which route to send packets along.
for example, you ask for a large file, the request uploads along through your default gateway. it arrives at the remote host, and it starts sending packets to its default gateway, and that gateway decides where to go next. you essentially have no control.
a discussion of serving large files is mostly identical.
on the other hand, if you have both connections coming in to the same router, and the router is smart enough to know about load balancing and bandwidth sizes and such, then it will automatically decide which link to use, on a packet by packet basis (which will give presumably much better performance than if you just statically specify one link for uploads and one for downloads).
what you want is for your OSX box to be that router. one problem with that is that if you want your whole network to benefit from that scenario, then they have to have your OSX box as their default router. you are basically giving away your mac to be a router, which i don t really think is the best choice.
the other problem is that OSX routed uses RIP routing protocol, which is not smart enough to understand different bandwidths. perhaps there are smarter software routers that you could compile for OSX, but i am getting fairly daunted at the thought of it.
you mention that you are hesitant about plugging it into your router because you don t want DHCP from the cable modem interfering with your network DHCP. you would also have that problem using OSX as your router (although it could be blocked). perhaps that DLink router can block DHCP packets too.
but Dlink. i don t konw what the specs of your router are, but if it is a home use cable/dsl router, then it probably doesn t even have RIP, making it entirely useful for my suggestion. what about your T1 router? can you plug the cable modem into it?
I sort of know what you mean, but I know not which device has a good upstream. Is it the cable modem or the cable modem?