You have a very low end system right there. You might consider buying a whole system.
For your computer I will suggest http://www.asus.com/Graphics_Cards/AMD_Series/EAH6450_SILENTDI1GD3LP/
because you have got a very low end system. And I don't am also not 100% sure it will work because you need at least PciE 2.0 or later to make it work. Check your PciE version on the motherboard specification first of all.