What board do you have now?
LGA775 is newer than S478, and it would leave you room to upgrade in the future. Socket 478 is going away soon. AGP is also, though probably not as fast.
Performance wise, the chips are the same, so that won't be a deciding factor.
If you go with an LGA775 socket board, you may have to use DDR2 and a PCI-E video card. I am not sure if there are LGA775 boards that support DDR and AGP. There could be, but if you are getting a new board then you might as well get the other new stuff on it. 😀
An LGA775 board with PCI-E slots would also leave you room for newer/faster video cards in the future.