Wireless Internet and Mac/Ipod

the iMac has a built-in wireless card, so all you would need is a wireless router.

You do lose a little speed, but not enough to be an issue. If you have six devices connected to your wifi network and they're all downloading or playing online games, you'll be in the same boat you are with a wired connection. Each device will only have 1/6 of the available bandwidth.

An iPod Touch needs wifi to connect to the internet.

I'd suggest buying an Airport Express and you'd be all set. All new iMacs come with wireless built in.

Get a wireless router and you will wonder how you ever lived before.

On the speed note, even a really slow wireless connection should give you over 10Mbit speed, and it's a lot more likely you'll end up using a "g" network, which is nominally 54Mbit.

Which is to say, way, WAY faster than your internet connection. Thus, there will be essentially no penalty in speed. Theoretically there might be a tiny addition in latency, but I just did a test; pinging a site from one computer connected via wired gigabit averaged 96.1ms over 11 pings, while the same site from a wirelessly connected computer averaged 97.2ms. So the difference is negligible, and either of those is way faster than I ever got with cable.

The only hit would be if you were transferring significant amounts of data between computers within the network (for example, if you had an in-house server); that would benefit from a wired connection. Of course, you can leave your wired router in for the computers that are talking directly to each other if you want, or get a router with a few wired ports in it (which is what I do--I have my desktop and server wired to my Airport Base Station via gigabit, but use my laptop wirelessly).