What could I benefit from replacing the AirPort Card in my '09 MacBook Pro?
I upgraded the built-in WiFi cards for my 2008 and 2012 Mac Pros.  It got me Continuity and the higher 802.11ac speeds, along with AirDrop (peer-to-peer WiFi).

To enjoy 802.11ac speeds (up to 1300Mbps if you're sitting right next to it), you need to also upgrade your router or access point (hereafter referred to as AP).  You also have to have a fast enough Internet connection (or have high-speed internal network needs) to warrant anything faster than 802.11n, as most ISP's don't provide connections over 200Mbps.  (802.11n specs are 300Mbps on 5GHz band of RF throughput, which yields just over 150Mbps of actual data throughput on good days.)

You don't get improved range without improved antennas; in this case your laptop's antennas won't change, so the AP would have to have better antennas than what you have now.  

-- And here's more nerdiness just in case anyone is curious... --

Speed is a combination of RF bandwidth (20MHz - 80MHz wide channels) and modulation (the way the bits are encoded into radio waves).  Higher modulation over larger swaths of RF bandwidth = awesome speed.

Because of WiFi regulations, the amount of power available for transmission is fixed.  Wider channels means less power (the power is spread across 80MHz channels in 802.11ac instead of 40MHz in 802.11n or 20MHz in 802.11g). The highest speeds are right next to the AP, naturally.

As the client radio (laptop or phone) gets further away from the AP (distance + obstacles), it will "downshift" from the higher bandwidths and modulations to the lower bandwidths and modulations until it eventually drops off.