I don't think you can get an i5 board that supports DDR2. As for the AMD x6 for DDR2 you need an AM2+ board that the manufacturer has released a bios update to use x6 chips but there dirt cheap £30+ in the UK.
It still depends on what you use it for and remember mobos for the i7 920 are nearly 3 times the price (in the UK anyway).
1156 is dead in 5 months so don't waste your money. AM3 is more futureproof and is cheaper. Also no DDR2 1156 boards.
I agree but I assumed they already had DDR2 RAM they wanted to use, I suppose unless they have a serious amount then its better to upgrade to DDR3 making the Price of an AM3 mobo £40+ but best to spend at least £70