I have a 30″ and it’s fine. If I had 2×23″ I wouldn’t have room for the broadcast monitor on my desk. Also, I never liked splitting a timeline across two monitors, and I never really needed to shove currently-unused palettes over to another monitor. With the 30″, I can see them all.
I’d use the money to get an 8-corre with 16GB of RAM.
I run 2 20.1 viewsonic wide screens. But I love having my timeline, effects/projects and main viewers on one monitor, and all my tools on the other, But I came from working on a 12″ laptop, to working dual monitor with that 12″ and a 20.1 and than finaly dual 20.1 on my macpro. and I love it, I love it sooo much it’s not even funny.
I’ve also been running virtue desktops (think spaces, but pre spaces) for like 6 months or more. essentially doubling my screen space. So i keep my craplications (itunes safari mail) in one set, and what ever I’m working on in another.
Really it’s up to you, some people love a single big monitor, I’d rather use 2 smaller monitors side by side