Q: MacPro 3,1

This is the machine i would like to upgarde to El Capitan

  Model Name: Mac Pro

  Model Identifier: MacPro3,1

  Processor Name: Quad-Core Intel Xeon

  Processor Speed: 2.8 GHz

  Number Of Processors: 1

  Total Number Of Cores: 4

  L2 Cache: 12 MB

  Memory: 4 GB

  Bus Speed: 1.6 GHz

  SMC Version (system): 1.25f4

I want to upgrade from  Mac OS X version 10.5.8 to Sow Leopard Mac OS version 10.6.3 and I get this message   Mac OS X Snow Leopard cannot be installed on this computer.. no explanation..

Do I need to buy more memory, a bigger hardrive?

Any help will be greatly appreciated