DDR2 Memory Compatibility

I'm building an Intel based desktop and am not sure what speed of memory I should use. Here are the basic specs:

Motherboard: Intel D945PVS (945P Chipset)
CPU: Pentium D 940 3.2 GHz 800 MHz Bus

The memory spec on the motherboard supports DDR2 667, DDR2 533 or DDR2 400 MHz SDRAM DIMMs. When I put these specs into Intel's barebones selector guide (http://indigo.intel.com/mbsg/Default.aspx) it comes back with "Dual Channel DDR2 400/533". My understanding is that the memory must run at least as fast as the system bus (800 MHz). I think this boils down to my lack of understanding of DDR2, but here are my questions:

1. Is this memory really operating twice as fast as the name, thus the "2" in DDR2? i.e. DDR2 400 is running at 800 MHz...
2. Why is DDR2 400 also listed as PC2-3200, DDR2 533 as PC2-4200 and so on?
3. Why is DDR2 667 (PC2-5300) not an option here according to intel?
4. Any general vendor recommendations for memory?

Thanks for your help.
JagerMAsked:
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MarkConnect With a Mentor Commented:
Just for more clarification of Callandors post. http://en.wikipedia.org/wiki/DDR2_SDRAM
Crucial offers all three flavors-->400,533,667mhz, of speed for this motherboard http://www.crucial.com/store/listparts.asp?Mfr%2BProductline=Intel%2B+Motherboards&mfr=Intel&tabid=AM&model=D945PVS&submit=Go

You also have to ensure you have the 0065 or greater bios installed for the Pentium D 940 3.2 GHz 800 MHz Bus processor you are running.

Here is a list of tested memory for your board, and the 667mhz ddr2 modules are included. http://www.cmtlabs.com/mbSearchResults.asp?sManuf=Intel&sMN=D945GTP%2FD945GNT%2FD945PSN%2FD945PVS&oSubmit=Search

Intel also states that 667mhz is supported http://www.intel.com/design/motherbd/vs/vs_mem.htm

As far as the configurator is concerned, when the 945p chipset is chosen alone, your board isn't even included in the results, therefore its not up to date as Callandor suggested
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CallandorConnect With a Mentor Commented:
1. Both DDR and DDR2 operate on the rising and falling edges of the signal; hence, they are actually twice as fast as the rating.

2. PC3200 is the total bandwidth, referring to 3.2GBytes/sec http://en.wikipedia.org/wiki/PC3200

3. It was probably too new for the selector guide.

4. I recommend Crucial, Corsair, Kingston HyperX, Patriot, and OCZ.
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JagerMAuthor Commented:
Great thanks for the info. Seems to make sense. I just needed to wrap my head around DDR2 and all the clocking that's going on.

The recomendations I've seen for memory speeds all seem to say that one step up from the matched root clock is the way to go. This system clock is running at 200MHz (quad pumped) which would match DDR2 400, but this would actually be slower than DDR 400 due to the latency of DDR2. It also seems DDR2 667 doesn't help that much because at that point it's really diminishing returns unless I could get to DDR2 800 (which I can't). So DDR2 533 MHz seems the best choice. Does that make sense?

One last point....should all of these DIMMS support the Serial Presence Detect data structure? I'm guessing they do in order to be fully compliant with the DDR SDRAM specs, but none of the descriptions I read actually say this. Is this just a given?

Thanks again.
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MarkCommented:
SPD is an industry standard these days as motherboard bios setup the ram settings automatically from this. I would be surprised if a memory module manufacturer does not have this capability in there modules.
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