Yeah, get the 775. Unless you consider AMD, but that is a whole 'nother discussion. Let us not get into that in this thread.
The most apparent difference between the two is the physical difference. In Skt 478, the pins are on the chip. In the Skt 775, the pins are on the board. The chip itself has 775 little contacts on it, and it lays on top of those pins. A clamp type deal is then brought down on the processor , clamping it onto the pins. This is suppose to be more efficient, but I think it just so you dont bend the pins on the prcessor. Especially the way Intel packs them. If you lift the little antistatic peice of plastic off of the chip using only one side when opening the container, it very easily bends the pins. That has happened to me numerous times, and i have spent HOURS meticulously bending those pins back in place.
And you dont have to use DDR2 with the 775, it is just an option.