well gigabit* ethernet is going to be the fastest speed.. but if every computer is wireless your speed will be limited by that.. now the wireless on a N series router which is probably what you would get if you move up to gigabit ethernet router is a faster wireless but only for the computers who have N series wireless cards.. and even then your only going to transfer at not much faster than the 100 meg wired connections.. with your current set up all wireless computers with that directly plugged in you'll probably xfer about 20-30 megabit per second mbps. if you up to a wireless N with gigabit router.. and still have wireless g cards you won't really see a change in speed.. the FASTEST way would be all gigabit wired connections.. that means your ethernet card on the computer cat-5 wires that can handle the speed and the router.. now you can mix and match.. but just remember that the slowest speed from one target device to provider device is the fastest you will transfer.
it seems like you have it, now if you add a wireless laptop then it will only slow transfer speeds that include the laptop:
nas to desktop speed: high
nas to media computer: high
nas to laptop: slow
laptop to desktop: slow
in that setup on your lan each xfer is three devices.. the device with the info, the hub and the device that is recieving the info.. so if all have ports that are gigabit and wires that can handle it then you get your fastest xfer.. and as far as wireless goes.. wireless N is the fastest i know of, havn't used it but i've heard tests that show performances of file transfers up to about 160 Mbps, which if you know anything about wireless it can't just send, there are a series of ackknowledgements that slow it down on actual speed.. they advertize over 300 Mbps.. real life after encryption between 100 - 160Mbps. so while you may go slightly faster then your old ethernet connection.. i doubt you'll notice much of a difference. while gigabit ethernet should blow it out of the water.