what frequency does ATV3 use? 2.5 or 5 or both?

ATV sees both. If your buying new I would recommend one that supplies both. I use the Apple airport extreme and would recommend it...

It largely depends on your circumstances which will be better for you.

2.4 GHz is more prone to interference from neighbouring networks and household appliances but is good over distances. 5 GHz is less prone to interference but is only useful over short distances and is easily blocked by walls and floors, even those of little mass such as studded walls etc.

You will have to manually set it up and configure the devices. As Winston mentioned there are pros and cons to both frequencies. Best to test yourself to see which works best in your environment.

Your netgear is a modem/router - correct? I have a stand alone modem and connect my airport to it. And then configure the devices through the airport...

This article may be applicable to your setup AirPort Express: How to join an existing Wi-Fi network in client mode - Apple Support

WWIK wrote:

So Apple TV uses both 2.4 and 5 GHz. I see that I have 25 neighboring wifi signals most of which match up with my current router. If i was to get a new router that offered 5GHz would my Apple TV automatically pick it up or would I need to manually set that up?

Those 25 networks are on 2.4 GHz, you won't be able to detect 5 GHz networks if your own network doesn't support it, whether or not any of those 25 networks are a problem or not depends on which channels they use and what strength they are, you may be able to determine this withiStumbler (NetStumbler for windows users).

Your Apple TV will detect the 5 GHz network automatically (assuming it isn't hidden) but you will need to log in manually.

You'll need a 5 GHz network before you can see other 5 GHz networks. It looks like the 2.4 GHz band is pretty crowded, do you currently have any problems with your wnetgear network.