Graphic card

E5700 is pretty weak for today standards, so I don't think that 7870 will cut well. I think if you want to play crysis 3 (which is a very-very demanding title) you have to upgrade everything else.

With that ol' LGA 775 processor, the 7870's performance will be bottlenecked thus crippling your hopes of seeing a dramatic increase in fps and graphics quality. Having such a card on an aged setup will be a waste of money. However, if you don't want to to build a rig, you can get entry-level gaming cards such as the HD 7770 or 650 ti.