Best display for reading text

Hi,

I'm taking courses online and need some help determining what specs I should look for in a monitor that's best suited to reading text as all the textbooks are online. Normally I have my monitor set to the lowest brightness setting but I've noticed that after a short time of maybe an hr I start to get headaches. My current monitor is set to 1920x 1200@60hz, its a samsung 245BW on win7, and I have two Radeon 6950's driving it, an I'm looking for a 27" monitor to replace it. Recommendations are appreciated, but the relationship between resolution, PPI, IPS, contrast, display and video card settings, and how those things affect text clarity is really what I'm looking to find out. Any help is appreciated. Sorry for such a broad stroke but it's difficult to find answers. Thanks