Given DDR3 prices, even if the memory speeds are the same, you'll be better off just replacing it all. Mixed RAM tends to cause all kinds of issues, and it usually takes some experience to know how to handle them.
Make sure your hard drive is decent as well. A slow hard drive can make an otherwise great system a total PITA to use.
Oh, and regarding what to do with the old RAM, I like to put them as Christmas ornaments. Every year, they become more and more obsolete, and it gets to be a reminder of how far technology has come. I have a few Pentiums and some 4MB DIMMs on my tree. </geek>
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