Yeah, I had that sort of setup in the past. It works fine, but the quality of the Amiga composite through the TV card isn't great at all, and I ended up opting for a scandoubler and monitor switcher instead. TV cards themselves work fine (I have one of the standard WinTV ones), and it served as my TV when I was at uni. I had some DIY add-ons to give me a remote control and an LCD readout of the selected channel
I'm not sure if there are any TV cards that will take an RGB input and be compatible with the Mediator, but if you could find one, it would make a massive difference to quality. It will still never match the direct connection to the TV however...