goes back to what you first said. i run my firewalls during the game plus yahoo and myspaceIM and i lose no proformance. but i have a nice enough computer to do it with
This is a bad idea and I cannot stress firmly enough it can cause problems. Just because it is acceptable to you, does not make it a good idea for everyone.
You cannot run any software with the CPU having to run it. When the CPU (regardless of the number of cores) has to run something, the game stops running. No matter the brevity, it does stop. Everytime a CPU core has to access memory, no other device, including other cores, can access memory. Not an opinion. That is the way it works.
Flotsom, the single biggest issue I have seen in software firewalls is people who like to log all the hits for every port in the computer. This is simply silly. The only ports you have to worry about are the which have LISTENERS attached to them. All the other noise reported by a software firewall is just noise to make you paranoid.
Logging means the disk will have to be accessed. Disk accesses are a pretty good guarantee that the game will take a severe performance hit. Manifestions include, pauses, freezes, stutters, warps and any other name you can attach which indicates non-smooth gameplay.
Bottomline is this; If you want the absolute best and smoothest gameplay, then you will do everything you can to make sure nothing is running in the background that is absolutely not required for the computer to operate.