Don't stare yourself blind on any one spec, but instead you should read reviews.
There are so many variables (shaders, clock speeds, memory bus, memory type, transistors etc.) that you cannot decide which card is better just by one single specification.
A 320 bit bus wont be much of a use if the rest of the specs are crappy.
The articles here on toms is a good way to decide which card to get:http://www.tomshardware.com/reviews/graphics-card-radeon-hd-5870-geforce-gtx-480,2621.html