1080p screen needed to match a 1920x1200 24 inch

the 1080 screen is a 16:9 ratio so its approximately .49x as tall as its diagonal, the 1920x1200 is 16:10 so its approximately .53x as tall as its diagonal, so to match a 24" screen that is 16:10 you will need a 26" screen that is 16:9. What is most likely to be tricky to match is the height from the desk, if their pedestals arent the same height that may cause you more issues than screen height.

If you are talking about eyefinity the 1920x1200 will only use x1080 if you pair it with a 1920x1080 screens.

Thanks for the quick responses, gave me the info I needed. I know I'd be limited to 1080 with my 2408, just want to see if I can save some money upgrading my two 19ers. I'm a little surprised that the 2408 is in short supply now, but I'm still considering to get two more. I probably shouldn't, but the thought of have a triple 24" setup is oh so tempting. Seeing as how common the 1080p res is becoming, and how much cheaper they seem to be, I thought it may still work well with what I've got.

+1 to jyjjy- if you are looking to game on 3 monitors with eyefinity, then you need to have the same resolution on all 3 displays, otherwise they will ALL default to the resolution of the lowest res display, so you'll have a distorted display on the 24" display you currently have. It is probably worth saving up the money you need to and getting 2 more 1920x1200 displays.