Well to start with you haven't got the latest version, that is 4.8.1201 that however, won't resolve what you want to do.
Why don't you just right click the avast 'a' icon, select Stop On-Access Protection and when you are done playing don't forget to Start On-Access Protection.
Avast really doesn't use that much RAM but if it is an issue I would suggest you add some more RAM. If you have 1GB or more RAM then what avast uses is minuscule by comparison.
The 4.8 version included a self-defence module to protect against malware that tries to disable the AV. See, Program Settings, Troubleshooting, Disable avast! self-defence module. That will allow you to disable services, again don't forget to enable it again, but I really think you should address the true problem RAM or the lack of it.