The link you posted is the G35 and G45 both IGP chipsets not the P35/45. You mean
this next page saying:
* PCIe 2.0 support
* ICHR10, ICHR10R (before ICHR9, ICHR9R)
Those are really the only notable changes but some vendors have thrown in some added features in their BIOS for the P45's, not to mention the P45 itself is built on a cooler more efficient 65nm process (P35 was 90nm I think). Basically, you can find P45 motherboards for the same price as P35's so it only makes sense to go with the P45.