The desktop will be worlds away from the laptop. I have my relatively new (close to a year old) with a dedicated video card which I 5 box on from time to time and it is like treacle compared to my desktop which is not that much faster power wise. The big weak link is the video card. When I look at my laptop performance, memory is at about 90% and CPU is about 60% but I still only get max of 20fps dropping down to 10fps on some spots. Desktop is a steady 60fps with the same settings.
Something that also hurts my laptop is that it's screen is 1900x 1620 (I think) LED display and so it has to run the main at that res (cannot find a way to drop the regular res to lower in Windows 7)
You will be really happy with the desktop except for the couch comfort factor