DDR2 667/PC2 5400 or DDR2 800/PC2 6400

I am buying an ASUS P5WD2 Premium mobo and a Pentium 4 3.40F cpu but i will be upgrading to a Pentium D Dual Core cpu so it would be great if anyone could tell me which memory would i have to get to ensure there are no bottlenecks at all, DDR2 667/PC2 5400 or DDR2 800/PC2 6400, i don't really care which is the cheapest just as long as there is no bottlenecks.

There is no reason to get anything faster than PC2-5400 RAM which can do 333 FSB at the 1:1 ratio. You'll be lucky to do 266 even with excellent cooling. Most PC2-4200 RAM will do close to DDR2-667 if you relax the timings. I have OCZ PC2-4200 that does DDR2-700 and Kingston PC2-5400 that will do DDR2-800.

Thanks for the quick reply but could you tell me what memory i would use if i wasn't going to overclock as this would give me a better understanding of DDR2 memory, would i have to use PC2 3200/PC2 4200/PC2 5400 to prevent a bottleneck.

It wouldn't make a difference. Your RAM speed is 2 * FSB. So if your fsb is 200, your ram is running at 400 mhz. You can get DDR2-667 ram, but if your fsb is only 200, it will run at 400 mhz. Now this is all at a 1:1 ratio. If you move your ratio up, you can get higher ram speeds. For example, my fsb is 275mhz. At a 1:1 ratio, its 275*1/1*2 = 550 mhz. However, I'm running a 3:4 ratio. So it becomes 275*4/3*2 = 734 mhz. This is with ddr2-667 ram. As you can see, it ran at both 550 and 734.
The speeds they give RAM are the speeds that they test it at. If you buy ddr2-667 ram, it has been tested at 667 mhz and it works fine. If you buy ddr2-400 ram, it has been tested at 400 mhz and works fine. That doesn't mean it can't run faster or slower. So if you didn't plan to overclock, I would suggest ddr2-400. I'd go with the cheapest because it will all depend on your fsb.

Now if you planned on overclocking and are gonna get some high fsb, you might want to get some ddr2-800 because you can actually hit that with a ratio. I doubt you'll make it that far...

Thanks that's helped me understand DDR2 a bit more, but i have one or two more questions.
Does PC2 3200 give a bandwidth of 3.2GB and PC2 4200/4.2GB and so on, and if the memory is running in dual channel mode is the bandwidth of PC2 3200/6.4GB and PC2 4200/8.4GB and PC2 5400/10.8GB etc?

The bandwidth is also related to how fast the ram is running. The faster the ram is running, the higher the bandwidth. So those numbers are really useless because your memory bandwidth will be different. Dual channel won't double your bandwidth either. Memory bandwidth also depends on your processor. AMD's built in memory controller makes it have much higher bandwidths. Just so you get an idea, someone on the forums got 7 gb/s bandwidth using ddr2-667 ram at 800 mhz.

Short answer, get some good quality PC2-4200 with halfway decent timings and you'll be fine. Guaranteed to hit 266 FSB. I can run the 3:4 ratio, but it don't do much to increase performance, so just run the 1:1 ratio. The Crucial PC2-4200 is a good overclocker and reasonably priced.

Here is a screenie of my 2X512 OCZ EB rev. 2 PC2-4200.

Thanks for the help lads.