I think the GOP would have to seriously screw up not to take the Senate. These are the seats that went Democratic in one of the strongest Democratic years in decades (2006) and likely won't stay that way. The GOP will probably gain seats in Missouri, Montana, Nebraska, New Mexico, North Dakota, Virginia, and Wisconsin. I think they have an outside chance at picking up Ohio, Michigan, and Florida. They will lose Massachusetts. Joe Manchin will probably hold West Virginia, but if the GOP takes control of the Senate, it wouldn't surprise me if he switched parties. He has always been more conservative than his party.
Nevertheless, the President makes judicial appointments, not the Senate. If Obama wins, he will reinvent the courts of appeals, and if he gets the chance, he'll do the same with the Supreme Court. Furthermore, unlike Republican Presidents, when Democrats make appointments to the Supreme Court, they consistently appoint social liberals to the Court, and though GOP Senators grill them a little and posture a little, they don't go to war to stop them. Accordingly, even if the GOP takes the Senate, I wouldn't expect them to make a serious impact on the composition of the Court.
In a second Obama term, Ginsburg would retire, and Obama would be able to secure the 4th liberal seat on the Court for the next few decades. If Kennedy or Scalia dies, he'd get a chance to shift the balance of the Court to a 5-4 liberal majority.
If he makes two more appointments, you'd almost surely see sexual orientation become a suspect class like race. Laws against same-sex marriage and adoption would be invalidated. The death penalty would probably be abolished. Most state and federal restrictions on abortion (parental notification, late-term abortions, Medicaid funding, etc.) would get tossed out.