i know 67% isnt 100%, but thats per DRAM clock... thing is, this framebuffer only really helps out at high resolutions or very high texture levels, the core speed is more important at realistic resolutions...
also, you must note that the 4870 doesnt compete directly with the 280... the 280 competes with the 4870x2, which has an equal bus width and much faster memory... so epic win right there!