Yes. It depends on game, resolution and what you call "smoothly". For example Crysis at 1680x1050 with all the eye candy on max might not be as smooth as you'd like, but CoD4 will be fine. You can just play Crysis at 1280x1024 if you want better fps, of course.
Is it too late to change the motherboard and PSU? With something like P5Q Pro/Silencer 750W (about $100 each at newegg) you'd be able to do Crossfire if needed.
True, 4 GB is almost as good as 8 GB in games. Read this:http://www.bit-tech.net/hardware/2008/07/08/is-more-memory-better/1