Well, if you've got the budget for it, I'd say you should go i5. However, that's not to say you should just scrape by with one of the cheapest mobos. In my opinion, if you've got about $350 to $400 to spend on the motherboard and CPU combined, a 750 and an upper-tier motherboard is a good choice. Otherwise, go for a PII 955 or 720 and a 790X motherboard. I'm partial to the GA-MA790X-UD4P (I believe it's called). What is your budget for the system and what do you plan to use the computer for?
Anyway, regarding sockets vs chipsets, at the moment there is only 1 chipset for 1156, that being P55. However, there will be more coming soon, though P55 will remain valid, so holding your breath isn't worth it. As for the actual difference, take this for example. LGA 775 has many different chipsets, offering different OC abilities, RAM compatibilities, etc. However they all (more or less) support the same CPUs, LGA 775.
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