Im not too sure about the cost of an adapter, but I will say any internal equipment sucks for desktops IMO. Poor signal quality and its easy to snap off an external antena (in the case of PCI linksys equipment) etc....
IMo the best shot for desktops would be an external USB reciever or a Ethernet bridge (which is a great investment because it will work with anything, Xbox, PS2, PC, Mac and you can add a hub and add multiple devices to a wireless network with basic ethernet)
Anyways, i would steer clear of the adapter or internal cards unless you arnt going to have them at long range, or only if you have some sort of flexible antena that wont break off.