Go for the 24", it's awesome I first thought the 20" would be just fine but, when I was in the store looking at both, my eyes kept going back to the 24".
I did this for two or three trips and finally settle on the 24", am I glad I did.
I have the first generation Alum iMac and it is a 24" 2.8 Extreme.
I have no idea what this poster is commenting on about his eyes going back and forth to view the internet!
I have no such issues!
Get the 24", it is a better monitor and color is great. Only problem I had was the screen was too bright when I first booted it, turned the brightness down and all is good!