If you already have a good router, just go for a straight Docsis 3 modem with 8 downstream and 4 upstream channels. Don't mess with gateways they tend to have a bad rep, what with bad range and crippled firmware from Comcast.
You don't mention which gateway you are looking at. Most that I have seen do have wirelless "N", but don't use 5ghz.
I've noticed internet performance for streaming video / mulitple devices is spotty lately, even with me buying the speed boost option. So I thought it was hardware related...would have been nice for Comcast to tell me about new standard.
As long as you don't have Comcast Voice service it is easy to use any approved modem.
If you have voice, EG says you need to check what telephony modems are allowed in your area and if they allow self install and so on. You need to pick a telephony modem in that case. (e.g. the Arris tm822g) But you need check he said.
I will ask one more question since you all seem more knowledgable than me on this....any settings / tips you recommend for when I get this Zoom modem and maybe altering my NetGear WNDR3400 router for the best performance?
Ah, missed your reply. I'd just say make sure all your network devices have wireless "N" cards or adapters. A rogue wireless "g" card or adapter will slow the network down. The rest is really what works for you...