NealR: What this means is that you need 4-6 times as many 2100MHz cell sites as you would 850MHz cell sites to cover the same area. So 2100MHz cell sites don't actually have more capacity, there are just more of them.
I have to mention that if a 2100 MHz network is limited to urban/suburban areas, where the existing network sites are relatively closely spaced, the additional 2100-only sites required are nowhere near 4-6 times the existing sites. AFAIR both Vodafone & Telecom are only deploying 2100 to built-up areas where the cost assoiciated with the smaller footprint make sense.