Yes. It is called Ad Hoc setting. Make sure that what you get is capable to be set as Ad-Hoc.
You do not need any thing else, just two Wireless Clint Cards, PCMCIA for Laptop USB for a Desktop.
Why USB?
Technology wise PCI card is probably slightly better then the USB.
However Wireless is depending on signal propagation.
For Desktop (or tower case) USB Wireless Client Card should be considered as the preferred solution.
PCI Wireless Client Card ends up been stuck behind the computer's case, resultant in obstructed transmission and poor performance. USB is connected to the Port with 2-3' cable allowing the Antenna to be placed above the System for better Transmission.