Just buy the newest one, that would be a N type router. If you want to go wireless to your desktop it will need a wireless adapter. There are lots to choose from but there are only a couple kinds: Internal(PCI) and External(USB normally)
USB - comes in 2 sub catagories: Dongle and corded. Dongle
is a thumb drive type device. Corded
is just that, a USB cord plugs into your PC and the other end plugs into the part that houses the antennae.
Internal - PCI
is a card with an antennae sticking out the back of the PC.
With all of these you plug them in and install some drivers and they should work just fine. I am not really recommending one over the other or those specific brands, those are just the first examples I found.
I will say this when my oldest brother asked me to get all his PCs online via wireless, the corded USB is what I chose. I liked the fact you could move it around if needed to get a better signal.