if you do plan on getting an e8400+ maximus formula, keep the chip you have now, because you are going to need a BIOS update to even boot with the e8400.
And about the p35, if you are not going to upgrade constantly, by getting the latest when it comes out, then go with p35. much cheaper, so you can spend more money somewhere else. But if you are going to be upgrading gpu's as they come out, I would go x38 for the pci 2.0 (which should be utilized by the newest stuff coming out) and the support for the 45nm is better. but the p35 is still a great chipset
and that p5k is a nice board. I dont like the placement of the IDE connector(wish they would move those to the top on all boards...) but that is a nice board. Once again, to support the e8400 if you plan on getting one, you will most likely need a bios update, so keep that in mind.