Yes there probably will be.
Intel's Woodcrest processors are tipped to replace the Dual Core G5s in the PowerMacs. They are 64bit dual core desktop versions of the upcoming Merom dual core mobile processor. They are more expensive than the up coming Conroe processors but Conroe doesn't support a second chip and so could only ever be a two processor system.
64bit systems support waaay more than 4GB of RAM so yeah, that's no problem.
EDIT: Whoops, meant Woodcrest