|There's a lot of mis-information out there regarding which RAM should be run with which CPU. Unfortunately, the use of quad-pump & double-pump technologies have muddied the waters & made things a bit more difficult to understand. If you stick to using the frequency rather than the advertised/theoretical FSB & DDR speeds, it should make sense. To reduce the quad-pump FSB speed to the actual frequency, just divide by 4. To reduce the double-pump DDR memory speed to the actual frequency, just divide by 2. Then you can compare "apples to apples".|
Generally, the CPU & RAM should be run at the same base frequency. This is known as having a 1:1 CPU:RAM frequency ratio. For the majority of Intel based systems (P1, P2, P3, Core 2 Duo), a 1:1 ratio will give best overall performance. However, the Pentium 4 (& it's derivatives) are highly inefficient, in fact, the P4 is said to be the worst CPU to come out of Intel. Anyhow, to help make up for the P4's inefficiency, it's best to run the RAM at a higher frequency than the CPU. There are two CPU:RAM frequency ratios that are considered optimal for a P4 configuration - 3:4 (for 400MHz FSB or 800MHz FSB CPUs) or 4:5 (for 533MHz FSB CPUs).
Since you have an 800MHz FSB P4 (which uses quad-pump technology), the base frequency is 800/4 = 200MHz. And since 3:4 ratio is considered optimal for an 800MHz FSB (200MHz freq) CPU, the RAM should run at 200MHz X 4/3 = 266.66MHz. Which RAM runs at 266MHz frequency? DDR-533 <-- there's your answer.