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A USB wireless dongle will do the job and shouldn't cost you more than the equivalent of fifteen British pounds. Don't bother with Wireless N standard unless your router can handle it or you're going to upgrade to a router that can.
An internal PCI card will probably cost the same amount but fitting in the back as they do, the signal has to fight its way through the metal casing to get to the aerial whereas many towers have front USB ports where the dongle could live.