Honestly, if you want to 'game' at high resolutions, get the 640mb GTS at least. The 320mb is great for 1280x1024 stuff and at those resolutions it matches the 640's performance. At larger resolutions is where the 640mb version excels. If you're serious about gaming at or above 1600, at least a 640mb GTS or I would reommend a GTX.
Otherwise, if your budget doesn't allow it, you don't game that much, or you plan on gaming at lower resolutions, the 320mb GTS or the 8800GT are the best values.
Dell and Samsung make some of the best large scale LCDs. Depending on size and price, its a toss up. I use 24" Dell's at work and my buddy has a 22" Samsung, both very nice.