Express is limited to 1 socket or 1 physical CPU, so getting 2 CPU's isnt going to help.
In 2012 it is also limited to lesser than 1 socket and less than 4 cores, but I dont know if this limitation is enforced with 2008.
It will all depend what your upgrade path is for the future as if you do plan to migrate to 2012 Express then just get 1 CPU with 4 cores or less if it is going to be a dedicated server just for SQL Express