The guys over at Viper's Lair have reviewed the XtremeDDR DDR400 True memory :
If you have a KT400 motherboard, you'll want some DDR400 ram. There's a lot of fancy overclocking ram out there, and we test if XtremeDDR should be one to consider.
"We finally reached the limitation with the memory running at 205. The system was stable at 205 MHZ, and was responsive to all commands. After 205 MHZ we hit a wall and could not continue. Limitations within the system were a definite culprit. The memory ran past its rated speed until the system prevented any further overclocking."