For a single card solution, the higher the bus the better the performance, generally.
For a multi-card setup, the larger the bus, the lower the scaling efficiency, as the bus size will effect how saturated the PCI-lanes will be. Two 128 bit cards (like the 5770 for example) will be better bang for the buck with near 100% increase in performance with adding a second card. But, if you start with a higher performing card (like a 5850) and add a second one, there is far less than %100 increase (closer to 70% in this example) in performance, due mainly to saturation of the PCI lanes, from its larger bus.