My daughter has a 2005 Dell XPS 400 computer. She was having problems which turned out to be with the memory. It wouldn't boot. The beep codes indicated a memory problem. I reset the memory sticks and that fixed it. But I noticed that it only had 512 mb of memory. I went to Dell's support website and using the Tag No. I checked for the memory type in the Parts and Upgrades section. It listed P6400 memory. So I ordered 2 gb of P6400. When I went to put it in I realized that the memory in the computer was P4200. So I removed the P4200 and installed the 2Gb of P6400. Everything seems fine. Now my questions. Will the two different speed memories run together? Will the P6400 run at 800mhz on this computer?