Just a real quick version on how I do mine:
1) Run Chrome
2) Install the Stylish plugin
3) Install the 'Twitch.tv Dark/Black Theme' and enable it
4) Popout chat, put it somewhere you like, at a decent size
5) Window Capture, Inner Window, with sub-region.
6) Chroma out the black. DapperKapper and MdKaP lose their hats, but chat is nicely readable.
You can combine steps 2 & 3 by just installing the Better Twitch.tv plugin instead (it has a chroma-black option under the gear icon on any popped-out chat) but I hate some of the other changes BTTV does to the site (like making the Old format content tiles transparent, which screws up a lot of the custom frames which no longer blend in properly).
Can head here and look at any of my past casts (http://www.twitch.tv/ferretbomb/videos
); Isaac runs it usually sits on a black background anyway, but Neverwinter it shows up very readably over the game source layer.
I've done a few tweaks to the Stylish theme CSS (keeping the scrollbars from auto-hiding, fine-tuning the background color to get the best key from it) but the above still works great.