Hei. I just recently bought mafia 2 to my 2011 mbp 13" 2.4ghz core 2 duo, 4gb ram that runs Nvidia 320M with 256mb and it barely works on low parameters. I keep the resolution 1280 x 800 because others looks just terrible on my MBP, but the game just lags and works very sloooow.
Almost the biggest thing that effects the game the most on that card (due to it's low power and it's lack dedicated RAM) is resolution. Trying to play at 720p resolutions with that card will cause it to slow down a lot as it just does not have the hardware compared to dedicated graphics cards.
Resolution is a very important parameter when gaming so you are not playing on low settings rather you are playing on quite high ones! Lower your resolution and watch the game speed up dramatically.
I consider these very decent parameters for such a game, how come is working so badly? The optimization of the game isn't so well perhaps? Should I wait for the patch and will it be able to help increasing the performance? I don't have anything to measure the fps, but it's not very high obviously.
The 320M is a low powered card it is not designed to run games it is designed to use low power and be able to run HD videos, mail and another standard programs users have installed and not high end 3D gaming titles.
To put the 320M into perspective the 27" iMac graphics card has 20 times the computational power of the 320M. In fact the AMD 4000 series card in the 21" iMac from early 2009 has about 4 and a bit times the computational power of the 320M card.
We have a few things we can do to improve the performance and with some help from NV & AMD (they have the game and have been helping us by profiling it on their cards) all this work should speed things up for you and improve your experience. All these will be in the patch but you have to understand the 320M is one of the least powerful cards installed in a Mac in recent years when it comes to gaming so it will never give the same experience as someone with a 2009 AMD4000 series card never mind the new iMacs with the AMD6000 series.
Edwin