The last official numbers from R* (this is about a year ago now though) regarding previous GTA's were:
GTA III - 13.5 million
Vice City - 14.5 million
Liberty City Stories - 8 million
Vice City Stories - 4.5 million
San Andreas - 21.5 million
(these were for all versions/platforms combined)
So it will outsell GTA III, possibly Vice City. I highly doubt it will outdo San Andreas.
Source: The numbers were released as part of a briefing to shareholders to get them to reject EA's offer. You could download the pdf previously but the link on wikipedia is now broken, you may be able to find them somewhere else. The number for San Andreas is in Guinness World Record Book as well.
Manchester United 2008-09 Season - Trophies & Records







