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).
CPU: e6600 @ 3.2ghz
RAM: 2GB G.Skill HZ PC6400
Mobo: ASUS P5B Deluxe
GPU: Sapphire HD4870
Soundcard: Creative Labs X-Fi XtremeGamer Fatal1ty
HDDs: 250GB WD + 750GB WD + 2x1TB WD
Monitor: Samsung 226BW
PSU: OCZ GameXStream 600W
Case: Cooler Master Stacker
OS: XP Professional SP3
Mouse/keyboard: Logitech MX518/Saitek Eclipse II
Speakers: Logitech Z-2300