If you installed the driver for your videocard with additional hardware you are getting the best graphics performance you can.

You get some extra software where you can tweak settings (for amd/ati is it called AMD/ATI Catalyst:

and for nVidia:

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It is important to note that it depends on what game you are talking. If it is native you probably need to depend on the ingame settings. If it is a game that you play with wine every game tends to have specific settings in wine that improve quality or speed but there are so many options it is impossible to tell. You need to rely on wineHQ for those games. And this, looks to me, beyond AU and more suited for wine forums.

Regnum Online (plays natively) has several ingame settings to improve quality over performance. Eternal Lands plays without problems at full quality.
A gane like Rose Online (wine) does not work well with my videocard and I need to keep it on lowest graphics settings. I got lots of black images (mainly floor) if I play with high perfomance.

All in all: Ubuntu is not really suited to play Windows games. But Valve is working in it ;)