Dual screens are nice for multi-tasking but not many games support dual screens (the only one i know of is supreme commander), so if you game alot i would go with the wide screen since it gives you an advantage (wider vision).
Im getting a another 17" monitor for my computer soon, but im hoping to upgrade to a asus striker and a couple of dx10 cards next year. Then i will get the monitor i suggested for you. Dual screens are nice but i think a 22" WS is better (for a gamer at least).