It matters more the OS being run. For a gaming build you would look at performance memory to see faster timings there. For 32bit versions of Windows you face the infamous 3gb barrier where more then 512mb is directed at hardwares and not available for software use. Once over 3gb a 256mb video card would see another 256mb of system taken for that.
Need 4gb? 2gb of ram works well for just about every one since CAD and other large programs would be the items that would demand more then that. Some top brands of memory are OCZ, Kingston, Corsair, Patriot is a newer brand name by an older company. and a few others. Type will depend mainly on application. For just a basic system you could go with value memory made by a good brand.