Gametrailers.com have an interview with Michael Hollick, the voice of Niko from GTA IV.
Hollick was originally drafted in to do just the motion capture, and at that point he wasn't even told which game it was for. But when the director heard his mock Serbian accent, he was brought in for the voice too. In the interview, he draws comparisons with Andy Serkis, who did the motion capture and voice for Gollum in the Lord of the Rings films, after originally only being a placeholder for the CGI.
Some of our affiliates: