The earliest we can expect to see Dual DDR boards for a P4 will be December, and it will be built on a SiS or VIA chipset. If you don't want to run on the bleeding edge, expect another two months for a revised release.
Intel won't incorporate Dual DDR until they release the chipset that will replace the 845 series. That won't happen until next March or April, depending on when the Prescott/P5 ships. As with VIA/SiS and the original 845, it might take a second revision for Intel to work out any bugs and open the chipset to overclocking.
I would recommend taking advantage of the mature 845E and 845PE chipsets available now, but buying two sticks of high quality DDR so if you do upgrade later you can carry over your memory without confict.
BHD