memory interface (that bit stuff) combined with the gpu VRAM speed determine a card total bandwidth. GDDR5 is faster than GDDR3. cards equipped with GDDR5 usually end ups a bit faster than it's counterpart that using GDDR3. for example if there are GDDR3 and GDDR5 version of 650 take the later. for your system i don't recommend for you to take 770 or 780. the highest i would recommend would be 750 Ti.