Yikes. That's almost a worst case scenario (but I don't have to tell you that with the problems you're having). If you decide to buy a new router, get one that is 802.11n. It's much faster and much more powerful. That may be all you need. The signal comes from the router to both your iMac and ATV, not from your iMac straight to your ATV. So you have to worry about what's between your iMac and the router *and* the router and your ATV. Good luck!
Edit: Oops, I just saw your latest post about the Apple Extreme. Yes. It is 802.11n with some newer stuff thrown in.