Memory
The speed and the amount of memory you choose go a long way toward dictating your system's level of overall performance. For a machine running Windows Home Vista Basic Edition, 512MB is the bare minimum. Double it to 1GB, and you'll see a big difference, especially when running multiple apps concurrently or using intensive image- or video-editing applications in Windows Vista Home Premium Edition and above.
The speed of the memory is also critical. Make sure the memory is DDR2 (double data rate) SDRAM. Midrange PCs can get by with 533MHz memory, though faster 667MHz memory won't set you back too much more. Written in one of two ways, memory speed is referred to by the speed in which it communicates with the CPU (1.8GHz, for example) or by its data bandwidth (5.3GB per second or PC5300). Though faster memory is available, today's chipsets do not support anything faster than 667MHz (without overclocking).