You might be confused by the terminology used by "Mb", as it's used for two different things.
- The Rapsberry Pi has a 100 megabit (mbps) LAN, which gives you a stable 12 megabytes per second (mb).
- A WiFi 802.11g is rated at 54 mbps which is about 6.75 megabytes per second, but knowing Wifi and interference you can only count on 50% band width, about 3.3megabytes per second if you're lucky.
- You get 802.11n 150 mbps and dual channel 300 mbps which is a lot better obviously.
- The newest 802.11ac promises something like 1000 mbps, using several channels and some crazy stuff. But to fully use it you need USB3 otherwise you might be lucky to get about 400 mbps on USB2. It would be best to get an 802.11ac router separate and keep in "ac" only mode, because if older devices connect it will slow down everything.
From my experience, if you just want to stream SD quality or highly compressed 720p then wireless N will be good enough. If you plan to stream 1080p mkv files, like myself, Wireless will not keep up, even dual channel N.
The recommendation for best experience is the get that wire installed or try and experiment with 802.11ac - You don't want HD movies buffering every 4 seconds!
-Edit April 2015-
I now own an 802.11ac triple antenna interface router (mostly for 802.11n support while not killing ac performance) I did some preliminary tests on a Pi and achieve very good transfer rates with one device on the WiFi. The problem though now, is range, its about half the range. I go into the kitchen and it drops into n (about 4~6 metres away downstairs) where n reaches outside, all the way on the road, about ~15 meters)