Can i combine 800MHz rambus ram with my 600MHz?

I am in the process of upgrading my RAM for my Intel P4.  I currently have a pair of 128MB PC600 (600MHz) Rambus RDRAM 184-pin RIMM Memory (256MB total).  600MHz being the most important part of that last sentence.  I have 4 RAM slots total and I was wondering if i was to purchase a pair of 256MB PC800-40ns (800MHz) Non-ECC Rambus RDRAM 184-pin RIMM Memory (to create 768MB total) if that would funciton fine alongside my current 600MHz memory.  Or do i need to be consistent and get more 600MHz rdram?  I was also wondering if, after purchasing the 2 sticks of 256MB, if the sticks need to be in a certain order.  For example: do the 256MB sticks have to go in "first" order and then have the 128MB sticks "second?"  ANY HELP or ADVICE is appreciated, thank you!
LeftHandedPantsAsked:
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CallandorCommented:
Yes, but like all memory, it will run at the slower speed http://www.rambus.com/products/rdram/rdram_faq.cfm.  Some motherboards are more picky about this than others, and you may find out how picky only after trying it.  If you want to play it safe, get the same speed memory.
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CallandorCommented:
The link also says you should put larger modules in the first pair of slots for better performance.
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LeftHandedPantsAuthor Commented:
Thanks a bunch!!
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pvercelloCommented:

I think this answer might be wrong.  To quote the FAQ that the Callandor mentions:

> Q: I have a RDRAM-enabled PC or motherboard with 800MHz RDRAM. Can I
> upgrade to 1066MHz or 1200MHz?
> A: Usually not. Whereas most systems can support slower RDRAM frequencies,
> the systems are not guaranteed to support to faster RDRAM frequencies.
> Please consult your PC or motherboard user's manual or manufacturer for details.

This was my experience recently when I tried to install 800MHz memory into my Dell Precision 420 which previously had 600MHz.  I got a "memory failure" boot condition.  So beware!

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CallandorCommented:
That's interesting.  I was looking at this reference, which led me to believe you could, but going up in speed beyond what the motherboard was designed for may be the exception:

Q: Can I mix RIMM modules of different frequencies, or ECC/non-ECC into my system?
A: When mixing module frequency or error-correction code (ECC), most RDRAM systems will default to lowest frequency among the inserted modules. In addition, if any of the modules do not support ECC, then most system disables the ECC function.
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LeftHandedPantsAuthor Commented:
Thank you for your fair warning.  Indeed, I don't like gambling with risks, but this risk has been worth it.  I have already installed the RIMM modules of 800mhz in addition to my previous modules.  Everything worked perfectly, and I now have a nice total of 768MB ram.  I'm pretty much assuming the system is defaulting to the lowest frequency (600mHz) as Callandor's insert suggested.  Essentially, I don't know how to determine if it is performing otherwise.  Do you know of any way to perform any such diagnostic tests on my RAM?  Also, this just popped in my head... Are there any BIOS options I might want to change/look at (regarding the ram)?  Due to the spur-of-the-moment quality of these questions, I realize these questions will not acquire anyone any points, so it's okay if no answers arise.  I realize, for the money i laid down for it, it might not be 100% efficient, but in the future 2 rimms of 256mb 800mHz ram could come in handy.  It was a birthday present anyhow. :)  
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CallandorCommented:
Everest will tell you how fast your memory is operating: http://www.lavalys.com/products/download.php?pid=1&lang=en&pageid=3

Don't worry about it not being optimal: if you're not crashing, it's working.
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