Bit-Tech published a feature titled CrossFire Comparison: Intel X38 vs. P35, which covers the various ATI CrossFire configurations available on Intel's 3-series chipsets. Here's an excerpt:

So I'm sure that one question on your mind is //what should I buy? Well, next generation cards from AMD and Nvidia aren't far away at all if the rumours are to be believed. However, they're likely to command a premium for being new, unless they are particularly competitive with one another. If you've set your mind firmly on buying two cards then at least balance this off with an appropriate solution - there are cheaper vanilla X38 boards out there if you can't afford an Asus Blitz, and if you're already prepared to throw £200/300+ on a pair of graphics cards, investing in a decent board to pair them with should be a necessary thing to keep in mind. But remember, simply overclocking or tweaking the system for more performance will negate some of the advantages of more PCI-Express lanes and should provide a better overall performance for your everyday computing, rather than just gaming.