does DDR400 mean it has frequency 400Mhz?

Kind of. <b>D</b>ouble <b>D</b>ata <b>R</b>ate RAM sends data at the opening and falling edge of each clock pulse, so if the RAM operated at 200Mhz it would act like one at 400Mhz. Technically your DDR400 operates at a real world frequency of 200Mhz, but acts like one at 400Mhz.

______________
OK it's your choice:
You can have the boat, or you can have the Mystery Box!
...Hey wait a minute! A boat's a boat, but a Mystery Box could be anything. It could even be a boat

in cpu-z, the frequency displayed is real world frequency?

Athlon64 Newcastle, 2800+, 640KB Cache, 1800 MHz
MSI K8M Neo-V, VIA K8M800 + 8237 chipset
Sapphire Atlantis Radeon 9600 256MB
DDR400 512MB RAM

If it states 400Mhz then no, if it states 200Mhz then yes.

______________
OK it's your choice:
You can have the boat, or you can have the Mystery Box!
...Hey wait a minute! A boat's a boat, but a Mystery Box could be anything. It could even be a boat