Your best chance is to get a Wireless Router that can be flashed with DD-WRT.
Place the Router next to the gaming Desktop, and configure the Router as a Wireless Bridge.
http://www.dd-wrt.com/wiki/index.php/Wireless_Bridge
Then plug the computer to the bridge with a Cat6 cable.
If the Bridge is far away from the main Wireless Router buy one that also have a removable external Antenna and use a High Gain directional Antenna on it.
Why this arrangement?
By Doing the Wireless reception through a wireless Router that act as a Bridge the Wireless is strong, more stable, and insert less lag than the Wireless that comes through a wireless computer card.
The computer under this arrangement works as a Cable connected computer and thus does not suffer from any Wireless trouble of its own.
The idea above is basically creating your own Media Bridge with additional flexibilities. Using a ready made Media Bridge is a matter of personal call. They are usually expensive and most of them "su**" performance wise.
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P.S. All of the above is just the principle.
It might be that the
2Wire G main Wireless can not work well to begin with and nothing wireless will work well with it for gaming.
In such a case you should Upgrade the whole system with an Additional Dual Band Wireless Router acting as an Access Point connected directly to the 2Wire Router.
