Sorehead, you're getting your computers memory and storage mixed up.
Memory, known as RAM is usually measured in Megabytes with 512mb being usual these days. RAM doesn't store anything permanantly. It's just a temporary store where your computer performs calculations and runs programs and the like. Having more memory means your computer can process more tasks at the same time, meaning it will run faster.
Your hard-drive, as Hammer says, is where your documents, files and programmes are actually stored. Unless your hard-drive is nearly completely full, it doesn't affect your computer's speed. Although like all things in computing having a bigger hard-drive is useful.
If you want to increase your compters speed, add more memory (RAM). It's relatively cheap and it's the best way to improve performance. It's quite easy to add more. Click here;
http://www.crucial.com/uk/promo/index.asp?prog=desktop2
If you want to buy some.
Also, having several programs running at the same time, as well as lots of programs running in the background and at start-up will eat into you computers resources. If you keep getting Low Memory warnings try closing down the programs you don't need.