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acruxksa · Rated XXX

I wouldn't say it's automatically better. It depends on whether you are going to use it or not. If you just pop your cpu into your motherboard and leave everything at default settings you could use DDR2-667. The only real reason you would need DDR2-1066 is if you planned to overclock. Even so, DDR2-800 would allow you to overclock your cpu to 3.6Ghz (9x400), so unless you plan on trying to make a run for 4.0Ghz DDR2-800 will suit you just fine. Given that you are new to building systems I would recommend you keep the DDR2-800 unless you can find some DDR2-1066 for about the same price.

Simply speaking, your memory runs at 2 x the speed of the system bus and your cpu runs at 9 x the system bus. This means that at default your system bus is 333Mhz so the memory is running at 667Mhz (2x 333) and your cpu is running at 3.0Ghz (9 x 333). When you increase the system bus to 400Mzh your ram will be running at 800Mhz (2 x 400) and cpu at 3.6Ghz (9 x 400). This is an extreme over simplification because most boards allow you to run the memory and cpu asynchronously which means that you could run the cpu at stock speed (9 x 333) and still run the memory at close to 800Mhz or 1066Mhz by changing the memory multiplier, running the memory faster than the cpu yields diminishing returns though, so it's not all it's made out to be.

Now your cpu will allow you to lower the multiplier, but not raise it. This means you have the option of using multipliers from 6 through 9 so theoretically you can pick a lot of different speed and bus configurations. like 3.2Ghz (8x400) with the memory at 800Mhz (2x400) like it was tested for. or with 1066Mhz memory you could even attempt something like 3.6Ghz (8x450) with the memory running at 900Mhz (2x450). There's nothing saying the DDR2-800 wouldn't be able to do 900Mhz, but you would be overclocking the memory as well as the cpu which introduces one more possible cause of instability.

These examples are an extreme over simplification of what's going on though because most system will auto detect the memory and choose a multiplier that gets it as close to the tested speed as possible. The issue here is that because your motherboard says 1066Mhz is an OC your motherboards bios may not automatically choose the proper setting for DDR2-1066Mhz memory and could possibly just run it at 800Mhz anyway. Meaning that you may have to manually configure it to get the 1066Mhz speed.

The faster the memory bus and cpu speed the faster the system. So all things being equal, a system running at 3.6Ghz with a bus speed of 450Mhz (8x450) would be faster than a system running at 3.6Ghz with a bus speed of 400Mhz (9x400). This is because the faster bus speed increases theoretical bandwidth available between the cpu and the rest of the system (memory, video, etc..). These speed differences are measurable and significant, although you would be hard pressed to notice them in anything but benchmarks.
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