Actually omni and sun-lamps can also produce grainy shadows if you don’t have enough samples, I generally would use at least 12 samples if you want to eliminate the grain, for very soft shadows upping that to 16 samples.
Or of course have a copy of 2.5 and grab one of the WIP exporters for Yafaray and Lux, which might be better because in BI you can’t accurately render say, frosted glass on a shiny floor (due to a limitation regarding reflections)