To record separate audio/video use Dxtory. It uses separate audio files. Also use an audio splitter like Virtual Audio Cable and put vent on a separate virtual speaker. I hate to reference a YouTuber so often but Jackfrags just does so many things right. Here is a tutorial of how to do what I just said.
[ame]http://www.youtube.com/watch?v=jw6YAoai8fQ[/ame]
Now for getting better recording performance people always recommend a faster cpu this is not always the truth. For a higher resolution/fps recording you will need a Hard Drive with @ least a 64 MB cache. I have heard good things about the 128mb cache Seagate constellation drives.
I use Sony Vegas Pro for editing the split audio, it's a great program but is expensive.