And good move on the power line solution. Now to fix the other areas. Google this:
How to use a router as a WAP?
Now you can put a WAP in the area you need more coverage. HOWEVER the router should be centrally located to the space it needs to serve. I'm sure you can see that now.
I hope you get some useful help on this, because I am in need of similar help. I have a 2-story with about 1100 sq ft on each floor. The basement is finished with another 700 sq ft or so. out router is in a corner upstairs spare bedroom and getting a decent signal on the main floor, let alone the basement, is a bear. For about 7 years I was using an old Linksys WRT54GS, but recently started streaming Netflix movies through out Panasonic BD player on the main floor so I upgraded to a Linksys N600. I can't honestly see much of an improvement from the old Linksys.
I also stream movies in the basement (Netflix thru Apple TV), but the quality in the basement is good because I use a powerline adapter down there. I am beginning to think that an improved router will not improve my situation.
Does anyone know if there is a solution to this problem or am I best served just getting a powerline adapter for the main floor as well? If another adapter is my best option, does anyone know if you can buy just a single one (Netgear AV 200)?