OK, you're wrong, but it's only because they aren't telling you what you need to know.
We'll start from the bottom. Say I have a batch of PC3200 and I start testing it to see how far I can push it. Some of the modules run stabley at 205MHz (DDR410), a few run stabley at 210MHz (DDR420), and a few run at 220MHz (DDR440). Since PC3500 speed is DDR433, I take those faster modules and label them PC3500. Does that mean they aren't PC3200? No, it just means that I have PC3200 modules that are able to run faster.
PC3700, PC4000, PC4400 are all doing the same thing, all are PC3200 modules that have the capability of running faster. Some use hand-selected chips, etc, but that doesn't change the fact that they're simply PC3200 modules with a lot of headroom for overclocking.
Consider the Crucial Ballistix PC3200 and PC4000. Say I have a batch of PC3200 modules of very high quality. I take one and test it, the lowest latency it can run at DDR400 is 2-2-2-5, so I call it super low latency, premium PC3200. Say I test it further and find that by loosening the timings to 2.5-3-3-8 I can get the modules up to DDR500. Now I have a choice, label them PC3200 Cas2 or PC4000 Cas2.5. I look at how many of each module type is needed to fill an order and split the batch accordingly. The RAM is all the same!
Say you're planning to overclock. You get PC4000, set it at PC3200 speed, make sure the system runs perfectly, then you can overclock from there.
A problem DOES OCCUR where you can't run dual channel or you can't run at DDR400 on some boards/chipsets, when you're using too many banks of RAM. Some boards will slow down a pair of double sided modules (4-banks), so you're best off with a pair of single sided moduels on those boards. This is significant to A64 chipsets, usually VIA but not excluding nVidia.
To make things even more confusing, Intel chipsets seem to PREFER having more banks, so double sided modules would be better for Intel platforms, worse for AMD platforms.
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