So my current setup is a 4 year old Mac Book Pro hooked up to a Dell 30" monitor (2560x1600). I also have a gaming computer hooked up to the same screen. When I want to use one or the other, I have to swap the cable.
I have an HD 5870 video card on my Gaming rig with a displayport.
The cool thing about the 27" iMac (2560x1440), is not only does the mini displayport on the back allow you to hook up a second display, you can use it as an input, so you can use your 27" display as a monitor. So I can hook it to my Gaming rig, and get rid of both the Dell and the laptop. Clean up a lot of space on my desk.
The computer was $1,899 (Apple Refurbished from there website). My Dell I can sell for $600 or so, and the laptop for around that. Not bad of an upgrade for a delta of $700 bucks.
P.S. If you have not seen the screen on the new iMacs, your missing out. They are the most incredible displays ever made. I could buy a 27" monitor of the same technology from Apple, but that alone is a grand. I think this setup will do very nicely. I will let you know :)