You have 2 options. Either replace your router with one fo the newer ones that have wireless or add a AP.
All of the wirless routers that I know of have 4 nic ports. I here people complaing that they have problems going through walls. That is mainly due to low gain (2dbi) antennas that come on routers.
I added a USR5054 AP to my system. It comes with 2 HI gain antenna (5dbi). With it I can cover my whole house on low power. The antennas along will cost you $30.00 each if you had to buy them.
If you are going to be working in the same room as the wireless router, you want have any problem.
I am able to cover my entire house with the power set on low. 5 walls in 60ft.
As for cost you pay about the same for a wireless router as you would an AP. I like them seperate, that way if you need to relocate (or raise) the ap to get better coverage you can with out moving all of your equipment.