How do I know what type of Gateway DX computer I have?

Okay, I am a computer moron. I have a Gateway series DX computer (It says so on the case) but I don't know which of the long list of models of DX computers I have. (see http://support.gateway.com/product/?cmpid=supportn... )

when I right-click to see properties of the computer I get this info

Model RS780 (which i believe is the motherboard)

Processor AMD Phenom 1900e Quad-Core processor 1.8Ghz

Memory 4.00 GB

System Type 64 bit O.S.

I am trying to determine what kind of memory I should get if I get more, and if I can figure out what kind of slots I have for maybe a new video card.

Yes I know it is old and not that great...I spend all my money on OTHER toys