We are upgrading RAM on a W510. There are 4 slots, currently 2 x 2GB installed.
Question is: can we install a single 4GB chip, or does it need to be balanced with 2 new chips?
You can mix and match the memory, although keep in mind it will operate at the speed of the lowest common denominator. I would not recommend it however as you will lose the dual-channel capability. 2x2G or 2x4G is your best bang for the buck.