Am I Memory Poor?

Hello,
I have a 3 year old Dell Dimension 6400 with 512 mb of ram. I am running
a memory monitor that I just found on Yahoo Widgets. It often reports that I
have between 15% and 30% memory available. Am I in a position where things
are slowing down because of it? I know you computer savvy people know about
these things. I know I am running a lot of stuff, but except for the
Widgets, I am running things I feel that I want.

Thanks,

Xray