What are compatible parts for this computer?

[xiang2011]

I work with computers but I've never built one myself and I don't have a working knowledge of Compatible Parts but my friend's mom wants me to look at their computer because it is old and runs slowly. She doesn't want to get a new one if she can avoid it, she wants to upgrade the current one.
Some system info:
Dell Dimension E521 - Windows Vista Home Basic
Processor: AMD Athlon 64 X2 Dual Core 3800+ ~2.0GHz
Memory: 958MB RAM
options for the PC.

I am planning to run a disk defrag and then I figured we could add RAM since it is anemic right now and Vista is sucking it dry surely. According to the manual the machine can accept up to 4x1GB RAM but I don't know what RAM to get to make sure they are Compatible Parts . Can anyone take a look and help?