That build should give around 50 FPS on high settings. Game is not that demanding (checked its specifications).
Try to run the game in compatibility mode with Windows XP SP3. Windows 8 is not officially supported.
I don't think that can be a problem, but no problem in trying. Also, try re installing the game. I was getting around 10 FPS in GTA IV on my PC, re installed the game and it was all okay with about 55 FPS on high settings.
Also you should install latest patch for the game. That should make a difference.