The ratio is of FSB speed (not quad-pumped) to memory speed (not doubled for DDR). So, at 1:1, 200 FSB = 200 memory (DDR400), 250 FSB = 250 memory (DDR500), etc.
At 5:4, 200 FSB = 160 memory (DDR320), 250 FSB = 200 memory (DDR400)
At 3:2, 200 FSB = 133 Memory (DDR266), 250 FSB = 166 memory (DDR333).
PC Ratings are as follows:
DDR266: PC2100
DDR333: PC2700
DDR400: PC3200
DDR433: PC3500
DDR466: PC3700
DDR500: PC4000