And don't forget, the 4850 is true single slot. The 4870 and all the high end nVidia cards actually take up two slots. So if you use SLI, you only have 3 slots left in a standard mobo, in fact more like 2.5 because a lot of mobos have one half width slot.
With a pair of 4850's you still have 5 slots left (okay, 4.5). Or just room for better airflow.