personally, i would advice on the phenom.
LGA775 is a dead platform, and it would really cut away upgrade potential to go that route.
i have a 550 myself, its very good. obviously, in threaded tasks it will be out performed, but as far as dual-threaded tasks go, its one of the best procs you can get.
what badtrip said is true. its possible to unlock to a quad (as i did) and the sucess rate is good. but its should only be a bonus, dont buy it soley on the basis of unlocking.