So a long cable isnt an option I'm guessing.
TBH I don't know, if both routers are of decent spec then I suspect there wont be any slow-down but I dont know for sure.
I think you should post this in the Networking forum, they would know a lot more about this than I do. If you need general system or water-cooling advice, I'm your man, but networking not so much.
http://www.tomshardware.com/forum/forum-42.html
Also just thought of something. Have a look at "Ethernet over Power". It uses power ports to act as faux ethernet cabling, just got to plug an adapter into the wall and into the router downstairs, you
should be able to receive a signal up in your apartment from the wall. Faster than wireless speeds, not as fast as Ethernet though.
http://www.youtube.com/watch?v=Sue1Zvmh8JA