The amount of random-access memory, sometimes called Ram, installed in a PC affects how many programs can be in use at the same time without the PC feeling sluggish. The most common versions of Windows XP and Windows Vista can't make use of more than 4GB memory and for most 2GB should be fine. The two most common types of memory are DDR2 and DDR3. The latter is expensive and only usable on the fanciest, high-end motherboards. The biggest factor governing memory performance is frequency (the MHz number). Ram rated at 667MHz, 800MHz or 1,066MHz will provide adequate performance (we've plumped for 2GB of 1,066MHz DDR2 memory). Only experienced users should buy memory that is rated beyond 1,066MHz, because the motherboard must be tweaked to use it.