Memory Upgrade Not Recog. In C Drive on XP

Hello,
I just upgraded with 1G on my Dimension] 4600. Restarted, etc., etc. System properties recognizes the upgrade - totals at 1.75GB. However, when I R click on C Drive, it only shows my original used space, free space, and capacity. I realized it when I tried to download photos and the process stopped due to inadequate free space. How can I get the full memory upgrade to be recognized? Thanks. Barb