Upgrade Mac mini's graphic card
If your Mac mini has a Thunderbolt port (which the mid-2011 models do), you should be able to add an external graphics card when those become available. Until then, there are no external graphics adapters for Macs except the DisplayLink USB adapters that @bmike mentioned, but those won't work for gaming.

An external graphics card is usually PCI-Express x16 and Thunderbolt only carries an x4 signal, so you won't get full speed out of that, but it's better than integrated (or maybe even just old) graphics. (Heck, many, if not most, PC motherboards only run x8 signals to attached x16 graphics cards, so they're not using the full bandwidth anyway.)