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For a 400/533/800 Mhz FSB mobo with slower DDR, what is cost-effective Socket 478 P4 choice?

I'm planning to build from a barebones system with a motherboard that has a 400/533/800 Mhz FSB for Socket 478 CPUs.  I have two 256MB ddr PC2100 memory chips I'd prefer to use, and I want to put in the fastest CPU that makes sense.  I assume I could use any Socket 478 800Mhz P4, but I don't know how the slower DDR SDRAM will affect performance.  Is it a waste to get an 800Mhz CPU (I wanted the hyperthreading) if I use the PC2100 chips?  Would a 533MHz CPU also waste performance potential with the PC2100 memory?  I want my optimal mix of CPU and memory, but since I'm building from surplus hardware I already own and only need to buy the CPU, I'm trying to make that my only new investment without paying for potential CPU performance I'll never see with the PC2100 "bottleneck".  Thanks much.
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justasking
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justasking
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CallandorCommented:
An 800FSB cpu will cause your motherboard to overclock your RAM, and very likely make it fail as it tries to run at PC3200 speed.  If your motherboard doesn't allow you to manually set the FSB, getting an 800FSB cpu with PC2100 RAM will render it useless.

That being said, I can't see getting a 533FSB these days, because it will limit you in the near future.  If you can't get the PC3200 RAM right now, getting a 800FSB cpu and running it at 533FSB would not be too terrible, if your motherboard can be manually set (you might even try overclocking the RAM).  When you can afford PC3200, then you can run at full speed.  By the way, PC2100 is the speed for 533FSB operation, PC2700 is for 666FSB, and PC3200 for 800FSB.
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zvitamConsultantCommented:
In My opinion you should be able to run even FSB800 CPU with PC2100 Memories providing you set correct timing in the BIOS.

However, using 266Mhz Memory instead of 400Mhz Memory makes little sense because the system will not reach it's potetial.

sell your old memory on ebay, and buy a new faster Memory that will match the performance of a recent CPU.
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justaskingAuthor Commented:
Increasing points so I can share among helpful commentators and request elaboration.

Listening to both of you, it seems that I should (1) confirm whether I can set FSB speed manually on the mobo (expected to be a PM800-M2 in a barebones from Wintergreen), and if so, get 800MHz CPU but force it to run at 533MHz until I get faster memory, and (ii) if I can't control the FBS on the mobo, then use the BIOS to (I'm ignorant here) slow the FSB speed (or the overclocking that the CPU might induce inthe memory?)that way.  Additionally, I should forget about PC2700 memory because that won't help---I need PC3200 if I use an 800MHz CPU.  Alternatively, I can get a 533MHz CPU, which will be obsolete sooner but eliminate these immediate challenges with the CPU, mobo and slower memory.

Please comment on/correct the above and indicate your confidence level.  Thanks so much from this non-expert.
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justaskingAuthor Commented:
Actually, I think I have 256MB of PC2100 and 256MB of PC2700, and would like to use both, in case that makes any difference.  I don't recall whether the likely mobo accepts 2 or 3 chips.
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CallandorCommented:
You first response is correct on all points - you understand the solutions suggested.  If you have a mix of PC2100 and PC2700, the system will only go as fast as the slowest RAM.  You can run with all the RAM at 533FSB, or remove the PC2100 and run at 666FSB.  All this is assuming that your cpu can handle these FSB settings (which an 800FSB cpu should) and that your motherboard will allow it to be set.
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CallandorCommented:
I think your motherboard can handle it - this looks like it, sold by TigerDirect as a Wintergreen system: http://www.ecsusa.com/products/pm800-m2.html
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