If the Soviet Union would not have broken up, they would have won the most medals in the Beijing Olympics. (H/T to reader Sera Jones and wisegeek)
There would have been some reduction or shifts in the medal total
Because a unified team would not have been able to send multiple teams to rhythmic gymnastics to win gold and bronze and would only have won gold A combined team would probably had stronger relay teams in track and field and in swimming. However, this could have been made up by having stronger teams in gymnastics for the men and women team results.
Reuters has an article which talks about China breaking up the old duopoly in Olympic medals. China would have had more gold medals than even a unified Soviet team. China will need to wait until 2012 to exceed the USA and a hypothetical unified team in total olympic medals.
China appears headed towards Olympic medal dominance