To do that, you'll have to get a board based on the VIA KT266 chipset, which fortunately for you, is mature enough now that it's a viable platform. I know of only one board that offers such a feature, and that's the Shuttle AK32. I don't know, however, if you can run both SDRAM and DDR-SDRAM at the same time, but I doubt it. If you can, you'll only be able to run as fast as your slowest RAM, though, which will be the SDRAM. Take the plunge and get all DDR if you're buying, though. There are no compatibility issues, and DDR is as cheap and sometimes cheaper than SDRAM, but offers a 5-10% performance boost in today's apps.
If you went all DDR, though, I'd recommend a different motherboard since hybrid boards like that tend to be a little flaky. I think back to the days of 30-pin and 72-pin SIMMs on boards, and then 72-pin SIMMs and 168-pin DIMMs on boards, and how unreliable boards like those were, but it may have just been the emergence of such technology and thus less mature.