CentOS 5 :: Memory Usage - How To Read Those Numbers Exactly At Top

My Centos PC has got 1 gig of rams. I'm running a webserver with 2 websites and a mysql service. also i need to start two other programs, which uses ~300-350 mbs/each. My memory usage without them is around 450-500 without them. It was 100MB~ free with everything i need on a sec ago. I've refreshed those two sites and it was ~50 left. SO what, site refresh for each user takes 20 megabytes or something? Is there any way to lower the overall usage without turning off mysql, apache and the two other programs? If there is how? And about number read, it says in the top command on mem like 4.4, 1.2, 2.0, etc.

What does that mean? I thought it's like 2.0=20 megabytes, but it doesn't add up to 450-500 then. And by the way, swap memory is 0kb. I have no possibility adding more rams except paying more 5 dollars a month for another 512. 5$/month is a lot for me (sounds funny)? Check out my country, you'll have to rethink. Especially being a student with no possibility to find a job). As for webhostings, i cannot find anything for free, that I could host my wehsites with ping less than 100 actually. Or <80 if say, then it's php memory limit is too low for my WP blog needs.