It looks like Mafia II doesn't support Crossfire or SLI.
Which means you can look at the single HD 5770 charts to get an idea where your video card performance should fall.
This game is really heavily skewed to Nvidia video cards due to it's use of Agea Physx
The CPU is also heavily impacted by Agea Physx settings. Mafia II review @ HardwareCanuks