I researched 20inch displays for my parents last year and they wanted something as close to the Apple as possible without the miserable refresh rate and cost. I settled on the Viewsonic VX2025wm. If you want a 24 inch, I'd recommend the newer bigger brother to this, the VX2435wm. I've been playing a few older FPS games on my 20 inch version, and there's no artifacts or tearing, and the image at native resolution is pretty close to seamless smooth. Color is very very good. The 24 incher is under $300 locally/retail, so it's a real steal for what it is.(yes you'll have to calibrate it, but that's true for any monitor)
The Apple looks a bit punchier because of the piece of glass in front of the panel, versus none on a typical LCD.