What was the percentage of registered voters that voted in the 2008 Presidential election and what was the final total percentage for each candidate?

There are currently 168,000,000 people registered to vote.

Of the registered voters, 126,478,160 voted in this presidential election. That would be 74.69% of registered voters.

53% of the vote went to Obama, 46% for McCain, 1% for other candidates.

I googled 2008 presidential election and the first link is the wikipedia article

United States presidential election, 2008 - Wikipedia, the free encyclopedia

Everything you asked is answered in there