I have a similar setup, though a much weaker GPU, and I'm not sure why you're experiencing this. I haven't played a game in XP since the day I bought Windows 7, but I don't think it's because of the OS itself. Windows 7 can be a bit difficult to work with when you're a new user, because it works on a completely different level than previous operating systems.
A few useful tips would be: never keep generic drivers
from Windows Update, always use the ones from the manfacturer's website; the generic Windows drivers are there just to make it work as a basic needed component, the full potential can only be achieved with original drivers. Also, ALWAYS install DirectX from the game installation CD. Though Windows 7 comes with DX11, it still needs DX9 files that the games were built for; if the DX setup says there are no components to update, you're sure that you won't have a problem with DX. Also, many games released prior to 2009 may (but not necessarily have to) be set to run elevated and/or in certain compatibility modes, so always check the Game Compatibility List
here at SF to make sure the game you want to play can run on Windows 7 (all the info and lists are on the first page).
As for Mafia 2 itself, i think that's just the way this game was "optimized"... Unless it's lagging A LOT (the only actual lag I encountered is right after leaving prison, when I'm supposed to meet with Joe; that small park in front of the building game me an amazing FPS decrease; it happens other times, but seems to go away as soon as I enter a car).