the P45 effectively replaces the p35. It was designed for Intel's 45nm Core2 processors. P45 is really good at overclocking. The Biostar P45 has the current world record for fsb, 725mhz and the Gigabyte p45 is really good at overclocking quad core processors.
P45s support Crossfire, they do not support sli. If you want SLI you have to get an nvidia board. The problem with them is that they aren't very reliable and don't OC as well as the P45s. The only Nvidia board I would recommend would be the 750iFTW, And I still think it is over priced.
Corei7 is Intel's new processor that will come out on the 17th of this month. X58 is the enthusiast level motherboard that will go with it (it replaces x48 and x38). Unless you have several hundred dollars to put towards the cpu and motherboard alone I wouldn't worry about them until next year.