I would advise investing in a USB WiFi Adapter for your desktop; although this is not to say that a PCI WiFi adapter would not work just as well.
However, if you ever plan to change desktop computers or get a new computer, a PCI WiFi adapter may not be compatible with the new desktop system, therefore you would not necessarily be able to re-use the PCI adapter, and you will be out the cost of the adapter.
However, if you get a USB device, you will be able to re-use that device with any computer with an available USB port.
Here are some samples of the type of WiFi adapter:http://catalog.belkin.com/IWCatProductPage...oduct_Id=179211http://catalog.belkin.com/IWCatProductPage...oduct_Id=272959http://www.dlink.com/products/?sec=0&pid=384http://www.dlink.com/products/?sec=0&pid=477
These models are just samples, you may be able to find one to suit your needs which will cost less than your budget is allowing for, but remember, you get what you pay for. If you can afford to spend a little more, go for the technology which gives you the best range and data transfer rate (speed). Remember, with the USB, part of the cost you are paying for is the portability factor in that you can move the device among multiple computers easily without much overhead - this means the useful lifetime of the product will be extended versus a PCI device installed internally on your computer.
NOTE: N-1 Wireless or Wireless Super G (MIMO) technology are going to give you the best range and signal as well as data transfer rate, they may cost more in the short run, but it will pay off in the long run. 802.11 a/g rated devices will work well, but you may encounter signals that are weaker due to the susceptibility to interference with the lower rated devices.