Welcome to the forums, Shiva81.
Question 1: I would definitely recommend a separate cable modem and router, NOT a gateway device. Invariably, these devices are compromised in terms of features and you will never get your money's worth out of them. Another thing is since they are a single device, Comcast has complete control of the firmware and it can only be updated by them and they are never timely about these things, or allow you to directly request an update. If the device breaks, you have to buy another device, which is usually more expensive than an individual cable modem or router would be to replace. I disagree with your assertion that there are more gateways then modems available, it seems to me to be just the opposite. I recommend the Motorola SB6120 as a good DOCSIS 3 modem (I own one, works like a champ). Do stay away from DOCSIS 2 modems, they are the last generation and while cheaper now they are also not capable of the new very fast speeds that becoming more and more common.
Since we don't know what modem you are using, we can't say if that model has gigabit ethernet or not. The manual for your model should say, if you don't have it, look it up online. Or simply connect it to a gigabit ethernet adapter with a good cat5e or cat6 cable and look at the adapter's port lights or settings to see if it connects at gigabit speed. The SB6120 mentioned above does have gigabit ethernet.
Question 2.1: I have an Apple Time Capsule 1TB, and while I generally don't like integrated devices much, this seemed like a neat piece of hardware and provided very nice Time Machine backup for my MacBook Pro's, I decided to try it. I have been a very happy camper with it, and yes, it's just an Airport Extreme Base Station with an internal hard drive. Whoever told you they don't work with Comcast was completely clueless, they (all 3) work just fine, as do the vast majority of routers. An AEBS with an external drive attached to the network or router will work just fine.
Question 2.2: Dual band means the router supports 5Ghz and 2.4Ghz wireless bands, which is a good thing if you are trying to cope with wireless interference, it gives you more options, as the 5Ghz band is less crowded. Do you need it? That depends on your individual needs, but it sure can't hurt. Most of your better wireless N routers are dual-band so it's pretty much a standard feature these days on better quality wireless routers. Not a gimmick, highly recommended.
Question 2.3: If you are going to stream video such as Netflix or something similar and want to do it wireless, don't skimp on your wireless router. Get a decent model designed for media throughput or high performance. I have no issues with streaming Netflix on my AppleTV or PS3 using my Time Capsule, so the AEBS should be just fine. If the HDTV has wireless, then make sure it has wireless-N as opposed to wireless-G (previous generation and slower).
Hope this helps.
