Upgrading a Shuttle SN26P

Hi everyone,
I just got my hands on this lovely machine and want to upgrade the RAM, CPU and GPU of the computer and was wondering on the best options available for me.

The model:
hxxp://global.shuttle.com/products_page03_simplify.jsp?PLLI=31&PI=90#

The Shuttle currently has:
AMD Athalon 64 3200+ (Socket 939)
2 x 512mb (PC3200) DDR400
NVIDIA Geforce 7900GS

My friend is selling me his Geforce 7900GS and hopefully I'll be adding this to the Shuttle + using SLI.

The official website lists the working components for the model:
hxxp://uk.shuttle.com/support_list03.jsp?PI=90

Would anyone be able to give me recommendations and what this little beauty is capable of housing?:)