For you hd 7750 suits best due to best low profile gpu as it fit in your case.
the problem is I don't think I have the 16 pcie.
From what I could find you have 1 16x PCI-E slot. From what I can gather it comes with a ATI card that would be using that slot so you would replace the ATI card with the new one.
But you will want to make sure your PSU can handle the new card.