|On a P4 system, 533MHz FSB doesn't actually run at 533MHz, it runs at 133MHz. The bus is "quad-pumped" so 533MHz is more theoretical speed than actual.|
When choosing RAM, you have to think in terms of bandwidth & bus speeds, not just bus speed alone. Remember that DDR & DDR2 RAM is "double-pumped", so when you see the RAM listed as DDR266 (for example), the actual bus speed is 1/2 of that, or 133MHz. And to calculate bandwidth, you simply multiply the theoretical bus speed by 8.
400FSB (@100MHz) x 8 = 3200MB/sec bandwidth
533FSB (@133MHz) x 8 = 4267MB/sec
800FSB (@200MHz) x 8 = 6400MB/sec
1066FSB (@266MHz) x 8 = 8533MB/sec
DDR266 (@133MHz) x 8 = 2133MB/sec (PC2100)
DDR333 (@166MHz) x 8 = 2667MB/sec (PC2700)
DDR400 (@200MHz) x 8 = 3200MB/sec (PC3200)
DDR533 (@266MHz) x 8 = 4267MB/sec (PC4200 or PC4300)
DDR667 (@333MHz) x 8 = 5333MB/sec (PC5300)
DDR800 (@400MHz) x 8 = 6400MB/sec (PC6400)
So in the case of this system with a 533FSB CPU with 4267MB/sec bandwidth, you'd need 2 x PC2100 running @ 133MHz in dual channel mode to have a 1:1 bandwidth ratio. The P4 tends to perform better if the RAM has a slightly higher bandwidth than the CPU, so you'd be better off getting 2 x PC2700 running at 166MHz in dual channel mode.