Games are lagging after upgrading to windows 7

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 Win7, but I don't think it's because of the OS itself. Win7 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 Win7 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 Win7 (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).

You may want to get 4GB of RAM... Also make sure PhysX is disabled in the graphics settings, that will KILL your framerate.