I think your best bet here is to use a USB WiFi adapter Vs trying to upgrade your internal unit.
Here's a good write up on the very latest 802.11ac adapters: [http://www.pcworld.com/article/2090556/tested-we-push-six-802-11ac-wi-fi-usb-adapters-to-the-limit.html|Tested: We push six 802.11ac Wi-Fi USB adapters to the limit]. Depending on your OS version you may not be able to use these in which case you'll need to checkup the older 802.11n adapters.