Which gpu should I choose?

pretty low end setup, single screen 24 inch monitor currently running on 8600gt
im thinking of getting the 560 gt because it is around my price range

or the 7870

i dont really understand all the differences with the specs and why they are different price when say i type in 7870 into ncix, there are different varieties at different prices

Can someone find me an ideal 560 and 7870 then the best between the 2 and why please. The 560 will suffice for my gaming needs but if you think i should go a bit bigger to last me longer then please provide an explanation. But I ideally want something the best bang for my buck.

Thanks!

oh yea, and my computer is like 5 years old, will i have to worry if my computer will take all current video cards? or do i need to check what kind of motherboard i have? its a dell inspiron 531