Sure, sorry, thought you were after an AC card specifically. IMO with the $6 or so price difference it would have made sense to go AC.
With both antenna working on that N card on 5GHz you ought to get around 100 - 140 mbps real throughput with a good signal. So will still be a large improvement over the current.
The '300mbps' is the negotiation speed between the AP and the Client. Wireless is half duplex, so cut that speed in half. Then there are overheads, and then add in lower signal levels, interference etc, the actual speed you get is not likely to hit more than about 140mbps with everything perfect - that is also assuming you are on 5GHz running a 40MHz or wider channel.