Windows vista 32-bit will only see about 3.3gb. For more, you need a 64-bit OS. No I/O is the fastest I/O. Vista will load and keep stuff in memory that it thinks you might use soon, so more memory is better. The XP approach was to page stuff out immediately to provide room for what you might need soon. If you add memory, get the exact same memory as you already have. Mixing types may not be handled well by some mobo's. In this case, 4gb might be best.