Why is RAM memory being sapped?

I started with 128mb of RAM running my Win98 with a Celuron 400 processor. While running Ms outlook I kept running out of memory. So I went to 384mb of RAM, But I am still running out of memory. For example I have been on line about 45 minutes now, and my little RAM program is showing I only have 144mb free. I have tried closing as many startup programs as I can and still I keep running out of memory. I am not sure how to look for a memory leak or stop one, if that is the problem. Hopefully some one can shed some light on this issue for me.
Thanks