Nice one. Building your own custom machine is definitely the best way, if you know what you're doing. I bought a couple of different laptops, which were crap for audio for different reasons, so in the end I went for this:
http://rainrecording.co.uk/products/livebook/
I know it's overpriced, but at the same time XP Pro is nicely tuned, all components are compatible and it's never given me any problems at all. I get low latency, with no f*cking around, and Traktor runs smooth as. :-)
(Also nice to have the support, as I've always got questions!)