I have a pretty nice Dell Latitude E6420 that I am using as a loaner laptop while I wait for my much nicer Dell Precision M4600 laptop to arrive. The E6420 has a Core i7-2720QM processor, but only had a single 4GB stick of DDR3 RAM installed.
The problem with this is two-fold. First, having only one stick of RAM installed can slow down memory access, since the memory controller in the CPU cannot take advantage of all of the memory channels that it has available. Second, 4GB is really on the low side for a new laptop computer. In mid-2011, I consider 8GB to be a more useful minimum (unless you are really not doing anything to stress your system). Fortunately, DDR3 RAM is quite affordable, so a 4GB stick can be had for $40-50. A quick trip to Micro Center solved that problem!
Looking at the Geekbench scores for the system with 4GB and 8GB of RAM, you see a nearly 50% increase in the Memory and Stream scores with two sticks of RAM vs. one stick of RAM. The overall Geekbench score improved by nearly 13%, which is not bad for less than $50.
More important is the improved ability to run multiple applications at once with 8GB instead of 4GB of RAM. This is much more noticeable in day to day operation.
Geekbench Score of Dell Latitude E6420 with 4GB of RAM
Geekbench Score of Dell Latitude E6420 with 8GB of RAM