Controlling the Yamaha receiver upstairs while you are downstairs is the easy part: The Home Theater Master RF-20/30 + RemoteBlaster is one example of an RF remote and basestation that can do it and is around $100.
The problem is that you are going to have to run some wires no matter where you place the amp:
1. If the amp is upstairs with the receiver, you'd need speaker wire from upstairs to the downstairs speakers.
2. If the amp is downstairs with the speakers, you'd need analog audio cables from the receiver upstairs to the amp downstairs.
3. If you use the powered zone2 of the receiver (instead of an extra amp), you still need the speaker wire from upstairs to the downstairs speakers.
You might want to check out X-10 controllers too. I'm not sure but you may be able to find an X-10 device that accepts line-level audio from the receiver and transmits it over the house wiring to downstairs where an x-10 receiver would receive it and send it to the amp over another pair of analog cables.
http://www.smarthome.com is one place that stocks a lot of that stuff. Worth a shot to see if any such thing is available.