Most monitor mfrs. recommend running LCD monitors at their native resolutions. If you run them at other settings, letters, words, etc. will appear blurry or indistinct. Try it and decide if you like it or not.
Your GPUs have more than enough power to try out your idea.
The max resolution on LCDs is almost always the native resolution I believe.
That resolution should give the HD4870 a good work out. To get any higher you would have to spend $1000+ on a 2560x1600 monitor anyway.