If you are only accessing the internet, it will not matter. Your modem to the internet is your bottleneck. I know in the US, the fastest home broadband connections only get up to 50mbps. Wireless G is 54mbps.
For sharing files and copying inside the network, it will not matter unless all the devices are connected via ethernet cable. The slowest ethernet connection is 100mbps with the fastest being 1gbps. But, all devices need to be on the ethernet to take advantage of the speed increase.
Example.
* I am on Wireless N - just more than 300mbps between all N devices.
* My NAS device is Gigabit Ethernet, plugged into my router which is also Gigabit.
* I wanted to copy over 1.5 TB of data from different devices to the NAS box.
* I plugged in my MBP via ethernet which supports Gigabit and did the copy.
* It was just over 3 times as fast as doing it wirelessly. Now, however, with everything copied to the one device, I am back to Wireless N.
