Mixing memory (same DDR2-667 & CAS 5, but different other timings)

1) What performance will I get if I mix modules with the same DDR2-667 and CAS 5 timings but possibly different tRCD, tRP, and/or tRAS timings?

2) And will dual-channel operate with the 2 modules within each pair the same, but the two pairs different from each other (in size and/or some timings)?

3) And will the setup likely be stable in a situation where I have no control of motherboard voltages or timing?

I am planning to upgrade an HP m8120n pavilion media center PC.

[currently fully as original: Intel Q6600 processor, ASUS "P5BW-LA" / HP "Basswood3G-UL8E" motherboard (P965-based, but doesn't seem to correspond to any of the boards in ASUS P965 archive, so probably a partial custom job for HP), 3 GB DRAM as 2 x 1 GB + 2 x 0.5 GB, DDR2 667 MHz, PC2 5300, 1.8 Volt Samsung chips, 1 GB modules have chips on both sides, 0.5 GB modules have chips on only one side; HP specs say board will support 2 GB modules, 8 GB total with 64-bit OS].

Not overclocked nor overvoltaged in any way, and BIOS doesn't seem to allow any such changes. (Doesn't matter; fast enough --- but I don't want to lose any significant amount of speed, or cause instability, when I increase RAM).

CPU-Z reports all of existing DRAM is 5-5-5-15-20 at 667 MHz.

I would like to upgrade DRAM to 6 GB (in conjunction with an install of Windows7 64), by retaining existing 2 x 1 GB, and replacing the 2 x 0.5 GB modules with 2 x 2GB ones.

Looking at Newegg's DDR2-667 2 x 2GB memory offerings, I see several with CAS 5, but no info on other timings. Corsair's has 5-5-5-18 timing, so its tRAS is clearly different and 20% slower than what I have now. The Mushkin does have the same exact timing and voltage as what I now have, but it has a heatsink, which increases the thickness. I could probably get away with a heatsink if I'm only replacing 2 modules. However, based on recommendations I have read, I'm inclined to choose Kingston or Crucial --- but standard modules from these only list the CAS timing, and no data sheet available on their company web sites. Kingston's DDR2-800 HyperX (KHX6400D2K2/4G) might be a possibility, because supposedly its SPD is programmed as 667MHz 5-5-5-15 to ensure bootup, but it's tested at 800MHz at 1.85 volts, and I wonder if it'll run properly at 667 1.8 v. Also, their datasheet is still labelled "preliminary" and dated 08/09/07, and they haven't replied to my email technical inquiry, which doesn't inspire confidence.

Thanks, in advance, for any answers or suggestions.
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More about mixing memory same ddr2 timings
  1. I think if you mix things up you have to use the lowest settings across all sticks. Likely to be the same with multiple banks, lowest common timings.

    Anyone else?
  2. If memory is rated 800@1.85v, 667@1.8v might well work. Best find 1.8v sticks though, or check you can return them if they don't pass memtest etc..
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