The motherboard looks good. You are correct. the Extra PCIe slot is for another graphics card, (it has to be IDENTICAL to the one you have, same manufacturer, chipset, model etc etc) and it doesnt really have any other use. For you, i recommend this memory
A-DATA 2GB DDR2-800
As for a case, a nice Thermaltake, or Antec case would be good bets. I have this one, and love it.
Thermaltake Matrix VX VD3000BWA Black Aluminum ATX Mid Tower Computer Case - Retail
As for power supply, this is a great, reliable, pretty quiet, and stable.
ENERMAX Liberty ELT500AWT ATX12V 500W Power Supply - Retail