Boy Did you luck out! It just so happens I have that same router which isn't currently in use.
The Upside to this is that when we get to setting things up I can Provide you with screenshots
which should make life easier.
OK so you have part of this right, so we're off to a good start. All you need to do Physically, is remove the cable
between the router and desktop. Unless you remove the wrong cable or your router is seriously crappy, then
you shouldn't have any issues with the Imac or Dell. As far as dektop goes, well you have 3 options.
1) Buy a Wireless G Card for the desktop Wireless G 311
(this makes choosing a card easy, but a few factors will determine performance)
IE; Distance between the router and Desktop and Any other devices involved.
2) Buy a new Router and Desktop Card that Support Wireless Draft N
This is more expensive, but you get a better range and performance (not necessarily needed)
3) Use option 1 along with a Ethernet over Powerline Wireless AdapterNetgear Powerline Wireless Access Point
( This one is totally up to you, please see the link)
I would generally recommend option 1; and option 3 if you get poor performance.
Option two is easier if the performance is poor, but more expensive.
By the way, please have a professional install the wireless adapter unless you
can't afford it or something. Wireless adapters can be picky upon install.
They may try to sell you a network setup, but this probably isn't needed.
The last one is up to you. I would only recommend the a network setup if
you need some very specific security or functionality out of your network.
So, go get the option that sounds best and have the Card installed.
I could help you with that but I take no responsibilty if something Goes wrong.
Looking forward to assisting further.