Here's what LH has to say:
When testing with games such as Crysis, we quickly discovered that the X48 chipset was able to deliver 10% more performance when compared to the X38, while it was slightly faster than the P45. While this was impressive, Crossfire is not all that well suited to Crysis, so we continued on and hit Devil May Cry 4, which works a treat.
Now when testing with the new hit title Devil May Cry 4 we found that a typical P45 setup is going to be 36% faster than a P35 system at 1920x1200 when running Crossfire Radeon HD 4850 graphics cards for example. The X48 was also roughly 6% faster than the P45 chipset, and I would suggest that this is a best case scenario for the high-end X48 chipset. That said, this did make the X48 around 15% faster than the older X38 chipset.
For the most part the X48 is just 5% faster than the cheaper P45 chipset
, while this chipset held roughly the same margin over the older X38 chipset. Therefore as we see it, X38 owners really would need to purchase an X48 motherboard to see any real performance gains, while the P45 is a massive step up from the P35 chipset when talking Crossfire performance.