If you are going to be using it for trading and viewing them at different angles, do not get the cheaper TN based LCDs. Their colors will fade and change depending on the angle. The Dell UltraSharp is mostly the better types. Dell monitors - Wikipedia, the free encyclopedia
Big is good, but look at the pixel counts as well.
Most of the newer displays are HD, so to reduce cost are dropping some of the vertical pixels (i.e. the one above is 1920x1080 vs older 24" monitors were 1920x1200). This is great for reducing the cost of monitors, but for trading, I prefer to have more vertical space. Now you can argue whether you really notice that the pixels are gone, but I know I do.
I use 30" monitors with 2560x1600 resolution (Dell 3007WFP). They are pricy new, but you can find them around refurb-ed.
Of course you can use lots of cheap monitors instead, but I personally don't want lots of monitors either.
You should be able to find Dell 3007WFP for $900, but you will have to look hard and keep at it, as they vanish whenever they pop up. Dell had refurbs on their site, but they are always not in stock.
As far as using a TV as a monitor, you are getting big, but you are getting bigger pixels, not more pixels. A typical HD display will still be 1920x1080, but it is 40" vs 24". That is good if you just want a bigger picture, but not so good if you are trying to get more resolution. One thing to note, is you need to have the desktop area to pull that off, as you will need to sit back away from the display to really see anything (this is somewhat true with 30" as well). All of these bad boys through off some serious heat as well, not to mention you can work on your tan
I am hoping that monitor prices may start coming down a bit at the high end now that Apple is coming out with a new 27" monitor for $799. It is 1920x1200, and replaces their previous 30" monitor.