I took a look at Comcast's current offerings here: http://comcast.com/internet-service.html
The only plans where your home network might begin to bottle-neck the connection are the 50 and 100 Mbps plans. The 50 Mbps plan might start to see some performance degradation across a busy 802.11g Wi-Fi network, and you definitely won't get 100 Mbps on that Wi-Fi. Your wired network should perform fairly well though, since that hardware more than likely runs at 100 Mbps.
If you're on a 50 or 100 Mbps plan, you might want to consider upgrading your wireless to 802.11n with MIMO support. To avoid RF interference, you might also want to move to the 5 GHz band. However, all of this requires that both your Wi-Fi Access Point and your client device(s) support them. Otherwise, you won't see any real advantage.
Chances are, the gateway device provided by your ISP only puts out a 100 Mbps wired connection so there's really no advantage to investing in Gigabit hardware for the wired side unless you plan on having a lot of internal network traffic.
To minimize Wi-Fi interference, and optimize your 2.4 and/or 5 GHz bandwidth, you should also consider running wired connections to all your stationary devices. (i.e.: Game consoles, smart TVs, etc.) This will make it easier for your wireless devices to achieve optimal performance, as well as ensuring better performance and stability for connections to your wired multimedia devices.