We can use registered/unbuffered RAM, but should I?

I have a 2.66 Ghz Quad-Core 2009 Mac Pro (4,1) which takes ECC (error correction) unbuffered DDR3 PC3-8500 (1066 Mhz) with thermal sensor.

I read that while registered/unbuffered RAM is one cycle slower it more than makes up for that by being faster when working with other RAM sticks. I planning on getting 8GB x 3 = 24GB to keep it at triple channel.

I read that registered/unbuffered will work. If cost is not a factor, should I hunt for and get registered/unbuffered? How much should registered/unbuffered cost over unbuffered, 25% more?

By the way, CL = 7 thus CL = 7 = 1066 Mhz?

Thanks for any help!

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Oh, also read that I can buy 1333 Mhz RAM and my Mac Pro would just down-grade the speed to 1066 Mhz. Is there any down-side to getting 1333 Mhz RAM?