One way or the other, you need to calibrate your displays for color accuracy. The extent to which either can be corrected is largely dependent on the capabilities of each individual display, as well as whatever graphics capabilities your computer has.
There are some test patterns and other manual adjustment systems out there; I have a DVD that puts up test screens of various colors and allows you to fiddle with the adjustments on a color TV set, to get it as close to true as possible, but the automated tools are much quicker and more accurate. The better ones also measure the ambient light in the room during the calibration process, and factor that into the new settings (if you use this, you either need to keep ambient light constant, or recalibrate if ambient changes significantly). Other, seemingly trivial factors, such as the color of the screen bezel and the color of your walls, can have a surprisingly significant impact on the colors you perceive as you look at your display.
I have a ColorMunki Photo, and if you live in the Austin, TX area, I will gladly loan it to you.
Since you use Costco for most of your output, you should talk to the techs there, and ask them what you need to do to your images in post processing, to obtain the best possible output from their machines and on the papers they use. They may recommend one color space over another, provide or point you to specific color profiles, etc. There's not much you can do about the accuracy or gamut capabilities of their machines; providing them with the most color-accurate image to start with is all you can do, and you'll need a well-calibrated display to get there.
One more thought: Since you were happy with the color performance (consistency with Costco prints) of your old Dell 20" display, if you still have it, you could hook up both it and each of your new displays to your PC in turn, and -- using color test screens -- adjust the new displays to match your old Dell as much as possible. Here's a good place on the web for color test screens:http://www.maccetera.com/screen/