Since it's a desktop (per OP's OP), I'd strongly recommend against a USB adapter, and for a PCIe cradle and a good, tested miniPCIe card. I just bought several of these: http://www.ebay.com/itm/130501831715?ss ... 1497.l2649
They were 10$ each and can take most any wireless card you like. I have a few old Airport Extreme cards to put in some of these, and some Intel 5300s to put in the other.
Decent miniPCIe wireless cards go cheap on eBay all the time, and you can pick up a really good brand, like Intel or Qualcomm Atheros.
Desktop cards, on the other hand, generally come from no-name-brand companies, often have poor driver support, and can be quite troublesome.
Here's a nice 5300 listing: http://www.ebay.com/itm/NEW-Intel-WiFi- ... 416bb0804d
You'd want UK listings instead of US ones, but does this sound like what you're looking for? Putting the card in the cradle is pretty simple, and driver support comes directly from Intel.
I've been here long enough that I think I can forgo a signature.