Good video editing/compression software should use dual-core to the fullest, as you say. Games don't currently, because writing separate animation, rendering, AI, physics, and game logic threads that all work together seamlessly is hard. What'll mostly happen with current code is one core will run the monster game loop, and the other one will handle the sound, data loading, and other background processes.
Eventually the new engines will figure out a good way to multithread, and then everyone else can copy from them. Likewise, there are a few special tricks that come with 64 bit processors, but mostly it's just about the larger addressable memory space. Still, no point in limiting yourself to 32 bit when 64 is available already.
As for video cards, I tend to try to find the best deal for the latest generation chip with the most RAM, because those two things will limit your gaming possibilities. The speed of the chip is important for framerate, but I'd take a newer, slower card over an older, faster one simply because it'll be compatible with new games longer. I use a 256 Mb X700 in my most recent computer.
The above may be choppy and difficult to parse. It is past 3 AM and my geekery is overwhelming my writery.