It seems that often when a best actor/actress Oscar nominee portrays a well real life character, the chances of winning are quite high. Of course there are some that don't win, but those seem to be fewer in number. I don't have the stats on it, but here is a list of Best actress/actor Oscar winners that played real life characters (lead or supporting roles). I am sure I forgot a lot of them.
Ben Kingsley - Gandhi (1982)
Adrian Brody -Wladislaw Szpillman (sp?) in The Pianist -2002
Jamie Foxx- Ray Charles (2004) in 'Ray'
Sean Penn- Milk (2008)
Colin Firth- King George VI in 'The king's speech' (2010)
George C. Scott - Patton (1970)
Geoffrey Rush - David Helfgott in 'Shine' (1996)
Philip Seymor Hoffman - Capote (2005)
Jeff Bridges- Crazy Heart (2009) (can't remember the name)
Forest Whitaker- The last king of Scotland (2006) - did he play Idi Amin?
Jeremy Irons- Claus Von Bulow in Reversal of Fortune (1990)
I am not sure if the actor who played Mozart's character in Amadaeus (sp?) won.
Will Daniel Day Lewis win this year for playing Lincoln, or will it be Anthony Hopkins for playing Hitchcock?
Those who did not win:
Russell Crow - John Nash in 'Beautiful mind' (lost to a fictional character, I think)
Joaquin Phoenix - Johnny Cash in 'Walk the line'
Leonardo di Caprio- Aviator
Denzel Washington - Hurricane (lost to Kevin Spacey's fictional character)
Meryl Streep- Margaret Thatcher in 'The iron lady' (2011)
Helen Mirren- Queen Elizabeth (2007)
Charlize Theron - 'Monster' (2004)
Hillary Swank - 'Boys don't cry' (2000)
Reese Witherspoon- June Carter Cash in 'Walk the line' (2006)
Sissy Spacek - Loretta Lynn in 'Coal miner's daughter' (1980)
Sandra Bullock - The blind side (2010)
Julia Roberts - Erin Brockovich (2001)
Cate Blanchett- Katherine Hepburn in 'The aviator'? (I can't remember the year)
Judi Dench - Queen Elizabeth in 'Shakespeare in Love'
Those who did not win-
Salma Hayek- Frida Kahlo (I don't remember who she lost to)
Michelle Williams- Marilyn Monroe (lost to Streep's Margaret Thatcher)
Cate Blanchett- Queen Elizabeth (lost to Gwyneth Paltrow's fictional character)
Meryl Streep- Karen Silkwood, Isaac Dinesan(Karen) in Out of Africa, and the Australian woman in 'A cry in the dark'