Careful on the RAM upgrade with Dell. I believe that they are finiky about what you put in, just any old SODIMM may not work right.
Bus speed is only one of several issues. Higher bus speed won't matter, but the computer won't work any faster either. You need to compare all the other specs too. Crucial is a good site to check out for that.
Check around, but last I checked Dell, they were the cheapest on RAM upgrade for my Latitude anyway.