If you can find a second GPU, it could make some difference. I’m using the GTX 285 for the interface and GTX 580 for image processing, and it’s a good combination for me (I’m also on Windows, with i7 CPU)
Like Laco says, some codecs are quite processor-intensive. It’s possible to render your material to a good-quality 10-bit intermediate codec, like Avid DNxHD. You can download it free from Avid, it’s a package named “Avid Codecs LE”. That way you can take some load off from the CPU.