The visual effects Windows 7 uses may be very pleasing to the eye, however they use up a lot of resources. If you like to play a lot on your computer this is an obstacle you do not need to face since you can disable compositions and visual themes upon starting a program. To do so, right-click the program's icon and select Properties. Browse the Compatibility tab and have a look at the options it provides:
The most functional options here are Disable visual themes and Disable desktop compositions. If you check their boxes, these two options will be triggered upon starting the program and deactivated again when the program is closed. This way you save a lot of resources which your games or other applications can grab.