The memory and CPU bus is not synchronized so there are different possibilities but for maximum memory we recommend following:

The P4 800 FSB processor has 6.4GB/sec for maximum Bus performance, with two similar DDR400 unbufferd non- ECC modules (2-way interleave), you also have maximum 6.4Gb/sec in memory bus, and this is a perfect combination.

For P4 533 FSB (Max.4.2GB/sec) processor you can either use two similar DDR 266 unbufferd non-ECC modules (4.2GB/sec) or two similar DDR333 unbufferd non-ECC modules (5.4GB/sec), but the DDR333 will run at 4.2GB/sec because of CPU max performance is 4.4GB/sec.