i'm just picking out the parts i need to build a computer. I think i'm going to get two ati radeon 4890 gpu's.
What do the graphics cards i get have to do with the monitor?
The resolution you run at. The bigger the screen, the more power you need.
I am using a 17 inch screen which runs at 1280 by 1024. a 4850 will be good for that
a 19 inch will be 1440 by 900 a 4850 will actually still be good for that
20-22 usually 1650 1050 you might think about a 4870 or Xfire 4830's or 4850's
and from there it is 1920 1080 you want the best out there which is like 2 4890's
You can get any card(s) you want at any resolution, but it will result in something called a bottleneck where your card can't produce it's full potential.