Hi khashu!,
You will need to get what is called RAM. These usually come in sets of 32Mb to 256Mb. To find out how much RAM you have, go START, RUN, and type in DXDIAG, hit enter. A Direct X application should come up telling you you're system info. In that list there should be RAM. This is just to check how much you already have. To find out exactly what RAM you need, read through you're motherboard manual. If you are English, I order most of my RAM from here:
www.special-reserve.com
If you are American, go down to your local PC dealer with you're mother board manual and they should help you from there.
Cheers,
Jack :D