without going into too much detail i cannot in good conscious recommend the powerline adapters. I used to do tech support for a company and one of their products were a rebranded set of those and the damn things were near worthless.
If you cannot run Ethernet from the basement to the 2nd floor (like i did) your best bet would be to set up a wireless repeater either on the first floor or somewhere on the 2nd that gets a strong signal. it will pick up on the existing network and rebroadcast it with much improved strength to enable coverage in areas it would normally not cover.
there are dedicated repeaters or access points and also some routers can be configured in this manner. If you ever wonder how large campuses, companies, and such have wireless all over the damn place that works near 100% all the time? this would be it...
