No luck for Sara at the draws for the Olympics. In singles she is 9th seed, but she plays former gold medalist (Sydney 2000) Venus Williams in the first round. They've played only once before, Williams won that match in two sets. You can see the full draw here.
In doubles, Sara and Roberta are 2nd seeds. In the first round they'll meet che Czech team of Cetkovska/Safarova. In the quarterfinal they could meet the reigning Wimbledon champions: Serena and Venus Williams. The full doubles draw is available here.