I have a 2.4 Ghz Macbook running 4 GB RAM. Some companies sell 4 GB chips to upgrade to 6 GB RAM. Has this ever caused problems? I am running VMWare Fusion and the extra RAM would be a big help.
You can probably buy a like 4GB chip on its own if you've already got a 2GB chip installed, but in either case, you're not going to be getting the performance benefit that comes from having a matched pair.