I debated this very purchase for 6 months...here is a summary of what drove my decision.
20.1": In going widescreen, 20.1" just wasn't big enough to truly enjoy the immersive experience of widescreen gaming, but the 20.1" model range does have some high quality and very well reviewed LCDs.
22": TN panels, but the sweet spot of price, performance and size.
24": Ideal, but I determined that 24" was simply too big for my needs...that, and the 24" LCDs were out of my price range.
I went with the LG 226WT, the highest rated LCD of the 22" TN crop. It is quite impressive for gaming, and the size is perfect, especially at the price point of $340. If you do go with the LG, there are three variants available. I have the 226WTQ, which does have ghosting issues in desktop applications, but I haven't noticed any ghosting in games. If you go with the LG, the common recommendation is the get the 226WT, not the 226WTX or 226WTQ. Best Buy has the 226WT, and I believe it is on sale this weekend for Mother's Day.