Networking – What does 300+300mbps on a wireless router mean

The quoted wireless bandwidth is the total bandwith for that the AP can provide, and is half duplex. So the AP can handle no more than, for example, 54mbps of data. It doesn't matter how that traffic is distributed, 54 clients each sending 1 mbps or 54 clients sending .5mbps and recieving .5mbps, or 1 client sending 54mbps or 2 clients, 1 sending 10mbps and recieving another 10mbps and another sending 24mbps and recieving 10mbps etc...
This is a rather naive example, in practice very few wireless networks can ever achieve their full quoted data rate. It is strongly dependant on signal strength. There are then various overheads, for TCP, IP and the wireless transport layer, including traffic that manages the connection even if you are not actively using it currently. These overheads include acknowledgments that need to be sent when data is recieved (and vice-versa).

Last thing is that some APs suffer badly when they are required to deal with mixed clients. My previous Netgear was a rangemax 240 which THG found dropped total throughput to less than 20mbps if a rangemax client and a standard g client connected at the same time. There are similar issues with supporting g and b clients, and although I don't know for sure, I expect that similar issues exist for supporting n and g clients.

1. Your Netgear WNDR3700's max speed in 5 GHz is 300 mbps with proper client support (meaning the client card has a dual radio chains that can handle it). Otherwise it will be 150 mbps or less, depending on which features the client lacks. These figures are theoretical and don't account for over-head, interference, distance etc.

2. This really depends on the walls and interference in the area, but your 780 sq foot apartment will have plenty of signal. I can get full signal on different floors.

In my opinion the biggest benefit of using both radios is that wireless is half duplex, which isn't something you notice unless you have multiple PCs streaming and online gamming. You may notice hiccups as all your wireless device fight for broadcast time on the channel...