DDR400 Motherboards do not necessarily have to have DDR400 RAM, DDR400 RAM is simply the maximum speed they support. Most boards will allow you to use a RAM/FSB divider which sets your FSB and RAM and different speeds. Or you might be using a 333Mhz FSB CPU in which case you can keep a 1:1 ratio of FSB to RAM.
As Flinx said we really need more information to be helpfull.
All DDR400 boards support DDR333 as well. In fact, with certain CPU's, you can't even run DDR400 at full speed.
The key is what CPU you use. If you have a 400 bus Athlon, an Athlon 64, or 800 bus P4, running slower than DDR400 RAM will result in decreased performance as the system will use a lower ratio for the RAM, with RAM running at 4/5 or 5/6 its normal speed.
With slower CPU's like the 533 bus P4, some chipsets will only allow a maximum speed of DDR333 anyway, even on a board that supports DDR400. So DDR400 would be clocked down.
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