I have a 800MHz FSB Pentium 4 computer that has 400MHz memory, Dual CHannel. It came with 2 128s(256 total), @400MHz. It has 4 slots. Last year I added 2 Infineon 512s, pc3200, and it worked, 1280MB@400mhz.(1024+256). Ive recently bought 2 more 512, to replace the 2 128s, and now it runs at 333mhz. Any suggestions??? Also, to answer some questions your gonna ask...The new memory is Kingston HyperX PC3200, for sure. Im aware of using PC2700s with 3200s will drop pc3200 to 2700 speed. But thats not the case. The kingston by themselves run at 400mhz. But when I pair it (HyperX, that is)with the infineons, its 2048@333mhz. Also, when i pair it with the 128s, its still 1280@333mhz. Ive also updated my bios to the newest version.