I want to add some more RAM to my Acer T180 desktop pc.

I have ran a Belarc scan and it shows-

1792 Megabytes Usable Installed Memory

Slot ‘A0’ has 1024 MB
Slot ‘A1’ has 1024 MB
Slot ‘A2’ is Empty
Slot ‘A3’ is Empty

But for the life of me I cant remember how to work out what kind of RAM I need and how much more I can get?

Any ideas?