I don't think pentium will be good enough. I'll also be watching movies and other stuff. Games like GTA IV might need the extra threads.
Its not the threads that are needed, its processing power. GTA IV is extremely unoptimised, its a terribly coded game, but fun. For GTA IV specifically, its CPU powered. However most games are GPU intensive..
Look, at $444, you can't exactly have a great rig. $600 is where price for performance really shows. At $400, its best not to build until you've saved a minimum of $550. I live in Australia too, and I understand the price upping here.
You could always go with a 965 Black from AMD, its much more budget oriented, and its a VERY good processor for the price.