We recommend you have at least a 2GB of RAM to run today's applications at peak performance. Everyone's needs vary based on your PC usage, gamer's, or graphic applications require much more RAM.
RAM doesn't make your PC faster, it PREVENTS your PC from running slow. Windows XP itself using 256MB of ram, Vista uses 750MB of ram, now load your anti virus, firewall, spy ware and there goes another 200MB of ram. Once your out of RAM your applications write to your hard drive to store temporary information. Guess what, the slowest part of your PC is your hard drive...yup big difference.
Do you have enough? Go to your control panel, system, in the general tab you will see how much you have. If you do not have 1GB and wish to continue using your PC for another year or two, or plan to get a new one within a year and give your PC to a relative, friend, new employee, or make it a second PC for your home buy more RAM.
A great website for evaluating what type of RAM you have, and the amount in each slot is www.crucial.com. Use the Crucial System Scanner and scan your system.
Installation of the RAM is at your own risk. Though this can be done by anyone not afraid to open up their case. Typically ram is held in by two clips one at each end, push these clips open to take out old ram. Then align the chip to the slot accordingly and push the chip in. The clips will automatically snap the RAM chip back in. REMEMBER: These are electrical components and stay grounded while doing any work and UNPLUG YOUR POWER CABLE before doing so. We also recommend getting matching pairs of RAM to maximize performance.