WI-FI card for a new standard

Your system currently supports 802.11n (Unibody MBP 15").

As to the just released 802.11ac standard its just to soon for any internal options for your current system (if they are ever offered). Your only sure route is externally.

Your systems external I/O interconnects (480-Mbps USB 2.0 ports) are just not fast enough to use an external 802.11ac WiFi adapter directly for dual link support.

If you can locate a USB 3.0 (up to 5 Gbps) ExpressCard/34 expansion card (up to 2.5 Gbps) you will be able to gain enough throughput to use a dual link connection 802.11ac device (via USB 3.0 interface). If someone makes a 802.11ac ExpressCard/34 expansion card that would be better.

FYI - The 802.11ac WiFi standard offers two types of connection a single link (500 Mbit/s) or a dual link (1 Gbit/s) where as 802.11n (a/b compatible) offers 54 Mbit/s to 600 Mbit/s depending on the band and antennas used and the connected AP device.

You will still need to have a dual link 802.11ac AP to gain the full benefit of the faster connection. That is assuming you have the bandwidth downstream to the internet to leverage it. Most peoples internet connection is not fast enough yet to take full advantage of this faster connection.