at stock 266Mhz:
1:1 = DDR2-533
2:3 = DDR2-800
With an e4x00 series @ stock 200Mhz:
1:1 = DDR2-400
3:4 = DDR2-533
2:3 = DDR2-600
1:2 = DDR2-800
As you can see, if you multiply the FSB by the ratio then double it you get the DDR rate:
266 * 1.5 (2:3) = 400 * 2 (Double Data Rate) = DDR2-800
Faster is always better, unless some component can't handle the speed. So, if you have a stock setup with stock clock speeds and DDR2-800 RAM, use 2:3. 1:1 will slow down your memory by 33%. If you're overclocking, you will probably have to lower your ratio in order to max out your CPU. See the Overclocking/CPU section and read wusy's guide.