How much RAM does your PC have? If you run Windows XP, it is probably in the range of 512 MB to 1 GB. Older Windows versions will do the job with less than that, but as soon as you execute demanding applications or more than one application at a time, anything below 512 MB will likely translate into performance bottlenecks.
This is a great article if you are still not using 2GB, but are considering it. They don’t take a look at anything higher than 2GB, but there’s not a true need for that right now. Check out the full writeup at Toms Hardware.