I agree that editing on larger monitor display is more optimal. I do my routine Photoshop editing on the 27 in desktop monitor and my quad core desktop computer. But for on-the-location editing or tether shooting, a laptop is more than adequate for the job. Laptop screens vary with different manufacturers. I find the Macbook pro and Sony laptop screens better than others, and can be adequate for Photoshop editing (you can always zoom in for detail works and use a Wacom tablet for finer controls). To get the best and accurate results, the monitor should be calibrated. This can be done easily using commercial softwares and products. If you do very large prints or use multiple monitors for editing with more complex requirements, then you may want to spend some extra cash for the professional GretagMacbeth products. But if you are more in the amateur/semipro category and do not have very specific requirements, then ColorVision's Spyder3 pro or Pantone's Huey pro will be okay. But the bottomline is that you need to calibrate the monitors whether it is for the large desktop display or the smaller laptop display. Once you do the calibration, the color, temperature, brightness etc will be more accurate. If you don't, then the prints will have variable results no matter which monitor you use.
As for RAM requirement, I really don't find a need to go beyond 4GB with any of my Photoshop applications, even when using the plug-ins. I usually try to get the settings right when taking the shot, so I usually don't do much postprocessing editing, other than doing some minor cropping and adjustments. Portrait touchups plug-ins, HDR and the chroma-key softwares all work quite well on my laptop with only 4 GB RAM. Batch processing is also not a problem.
But if you do intense HD video work, then 8 GB may speed up the process a bit. My Sony laptop with i7 processor, 4GB RAM and 1GB GPU works okay with routine consumer grade HD video editing. For 3D video, animation and gaming, you may benefit from 8 GB or more.
So for cost-effectiveness, you can consider getting 4GB with option to expand to 8 GB if you do find a need for it later on.