[Aside: DDR RAM is SDRAM. SDRAM is Synchronous Dynamic RAM, which nearly all memory has been for many years. SDRAM replaced EDO RAM starting in 1996.]
They won't exist. P4's were bad enough when they ran on PC2100 that I doubt any board maker would dare put SRD . If you remember, the P4's took a huge performance hit when you took away RDRAM. Dropping back to PC100 or PC133 RAM would probably make the system perform worse overall than a PIII system.
I suggest picking up an AMD board and processor. Something like the BioStar M7VKS which has both SDR and DDR banks.