I myself would go for a LGA775 processor. It's not that they are any better exaclty, but the upgrade path for the socket 478 processors is dead. 3.4GHz is the fastest you can install. (You can overclock to higher speeds of course) but the LGA775 processors come at speeds of up to 3.8GHz.
So for more flexible CPU support for the future I'd say go with LGA775.