fmw is right and wrong, motherboards do have upgradability, for instance i have a socket 775 mobo with a 65nm dual core processor, i can go up to core2quad extreme with it, i have 2x8600GTs, i can upgrade to better video card(s), I have 2 HDDs that aren't in RAID, i can get another HDD or 2 and put them in RAID, the only thing i can't upgrade is my RAM, the most my mobo can handle is 8GB PC-6400, and that is what i am on.
He is right saying that new chips come out that make old ones not so good (not obsolete though) in comparrison, but still fully able to do what you need them to, my system isn't an i7 system but still does everything on full for example
He is also right in saying though that you must first choose the chip, i would recomend intel because they are faster and more economical at 45 nm (use less power).
AMD are the cheaper option but aren;t as powerful and the architecture isn't as efficient (takes more time for an equally clocked processor) as an intel chip.
Saying this though, a phenom II quad will blow a core2duo out of the water, it does depend on the chip but what i said above is a generalisation.
Also you must decide on your video card, if you want nvidia, you will want a board with nvidia chipset for better support, same for ATi