Originally posted here by HTRegz
The choice is yours to make.. depending on if you have the money... bigger is obviously better.. but if you aren't going to use it.. why waste the money.. unless you want the bragging rights.

The big thing to remember is that the speed of your FSB and the speed of your ram need to go together....

DDR400 will run at full speed on a 800Mhz FSB.. but only 333Mhz on a 533Mhz frontside bus and only 266Mhz on a 400Mhz FSB
DDR333 will run at full speed on a 533Mhz FSB, but only 320Mhz on a 800Mhz FSB and 266Mhz on a 400 Mhz FSB..

So make sure you buy appropriately if you go with the 800Mhz FSB but then buy DDR333 your ram will only operate at 320Mhz and if you get the 533Mhz FSB then DDR400 would just be a waste of money.

[EDIT]
I guess I should have mentioned that this applies to the Intel i865G and i865PE boards for certain.. I'm unsure on other motherboards
[/EDIT]
Same thing applies with SIS chipsets as well. I was rather puzzled when I upgraded my PC at home with a new m/b (533 Mhz FSB), P4 processor & DDR memory. I'd asked for DDR333, but actually got shipped DDR400 instead. PC didn't boot, but on power off/power on it booted correctly with a warning message saying some settings had been changed on a temporary basis - some sort of 'safe mode' I guess. Pretty good of the BIOS to figure this one out

I managed to fix the problem permanently by going into the Frequency/Voltage Control options in the BIOS and choosing a ratio of 5:4 so that the memory ran at the correct speed relative to the clock frequency (i.e. DDR 333 rather than DDR 400).

Trouble is the BIOS asks the DDR chip what speed it runs at, and when it gets a response of DDR400, it just crashes, but it does remember this for the next time you boot, and corrects it on a temporary basis ...