Is there an upper limit of RAM?
Before you splash out on a bumper pack of DIMMs, check you’re not wasting money on more RAM than your system can use. XP won’t recognise more than 3GB, so there’s no point adding extra.
With Vista, the situation is more complex: 32-bit editions can address up to 4GB of RAM, but that has to include not only your system RAM but also the memory in your graphics card and other system resources. If you install a 4GB DIMM, you’ll find less than 3.5GB available to the system. Given the diminishing benefits of adding memory above 2GB, you might want to consider saving money and sticking at 3GB.
64-bit editions of Vista can take advantage of far more RAM – the 64-bit version of Home Basic will take up to 8GB, while Home Premium supports up to 16GB and the Ultimate, Business and Enterprise editions can accommodate a massive 128GB of RAM – though good luck finding a motherboard with enough DIMM slots.