I've got a 2 x 2 GB kit of the Mushkin HP 6400 (800Mhz) I'm reviewing. They are currently doing 902 Mhz 5-4-4-12 @ 2.25v memtest stable on P5B Deluxe 1004 bios. Going to push them higher but just installed them and loaded Vista x64, not a problem - just make sure to enable memory remap so the OS can see 4096MB. Someone asked about Vista x64 earlier - yes they work fine with Vista X64.