The MacBook Air isn't exactly known for its raw graphical might, and its super slim form factor means integrated Intel HD Graphics is as good as you're going to get -- unless you spend some time and cash to build an external graphics processor, that is. Tech Inferno forum user Kloper has put together a rather in-depth guide on building an external GPU setup which connects to the MBA via its high-speed Thunderbolt port.
All told, the setup isn't exactly cheap, as you'll need to drop around US$250 in adapters, cables and a power supply, plus the cost of a GPU which can run several hundreds of dollars or more. Oh, and you'll need to have Windows 7 installed on your notebook as well.
The Frankenstein'd rig isn't even remotely portable, and it's only good for gaming when at home, but the setup described -- which uses an Nvidia GTX 570 -- manages to crank out some impressive benchmark FPS measurements using rather demanding games, like BioShock Infinite.