It's a common problem actually. Wireless isn't all it's made out to be, and you'll struggle to get 20 MB on wireless (there's lots of overhead on the wireless protocol).
I'm not quite up to scratch on the more recent wifi developments, but I'm sure someone that does will be along shortly. I wouldn't hold my breath though. If you really want 20 MB, I'd recommend good old ethernet
Oh:
to the forum btw.