Ram question

It sounds like you have two different things going on.
1. You have low hard disk space. The way to fix this is to buy a new hard drive to add to your system, or to replace the existing hard drive.

2. You have a small amount, 2 GB, of RAM. The way to fix this is to buy two memory modules of 2 GB each, raising your total system memory from 2 GB to 5 GB.

The type of RAM you have is very old and finding anything larger is going to take a lot of searching. The other issue is your board might not support larger quantities of RAM so 4 512MB sticks might be the limit. So you will either have to search for the model online or post it here and someone can let you know.