Yes the GDDR5 makes a difference, the memory clock data rate is multiplied by 4 on GDDR5, on GDDR3 it's multiplied by 2.
So even with lower Memory Bus Width
the HD 4870 has more Memory Bandwidth
than the GTX 260.
On many reviews the card was run on resolution up to 2560x1600 and nobody reported the 512 Mb WRAM was a bottleneck.
Read more at AnandTech
PS My choice is still HD 4870