If you plan on upgrading to a higher bus speed later, get the PC2700 as a second stick now...and replace the PC2100 when you get the higher bus speed CPU. No, you don't have to throw it away, you could put it on ebay or something. If you know how to advertize it, you'll probably get more than you paid for it...unless you bought it off eBay.
<font color=blue>You're posting in a forum with class. It may be third class, but it's still class!</font color=blue>