OK, so I did some fiddling, and I finally got things to work 'mostly' how I want...
I put the media-PC (wirelessly) on the Linksys router. No issue there. It will access Netflix, Hulu, et al with aplomb.
By default, its gateway is the 25 Mb connection. It will never 'see' my home network (the Asus), so I can game and stream media at the same time.
I installed a 2nd NIC in the download-PC. The 1st NIC is connected to the Asus router with a static IP, and with no Gateway or DNS addresses set,
so all the devices on my home network can still access all the shares on that PC. The 2nd NIC I connected to the Linksys router, with full settings
for Gateway, and DNS, so any internet traffic passes through that connection, and does not harass the Asus Gateway at all, so my main home network
is unchallenged when uTorrent and Alt.Binz are working that 25 Mb.
I verified that all devices are using the correct Gateways with plenty 'ping' and 'tracert' commands, so I'm good to go.
...oh and the media-PC can still access all the shares on the download-PC, by virtue of them being on the Linksys router,
which is what I wanted. I didn't really need devices on my main network to 'see' the media-PC at all.
So now I can download whole day, whole night, and still shoot-up Notnice and dem when ah ready without issue.