Check your motherboard specs carefully. I suggest you sell the 512 stick on your local craigslist or keep it for a spare. Don't mix it with 1 or 2 gig sticks. If your board is old, 1 gig per slot is probably the limit. The speed increase will depend on your operating system and other factors. If you're planing on upgrading to vista or 7, then it's mandatory. For xp, maybe a 50-75% increase in overall speed is realistic. You'll just have to try it.