I currently have only one tab open and Firefox is consuming 328 MB of RAM and 349 MB of my page file, both down by around 100 MB since I had lot of tabs open. That's what BAILOPAN calls "aggressive caching".
If this computer is only used for Counter-Strike hosting and you're behind a router, antivirus software is rather unnecessary. Then again, I don't consider it worth using on my general purpose machines either (XP/Vista).