importance of matching FSB and memory speed

this is probably going to be considered a newbie question, but the systems I normally work with are preconfigured.  Basically I'm wondering how important matching FSB and memory speed is.  I'm looking at building a machine and I've got an atx motherboard, atx case, and a 450 wt power supply.  The motherboard supports the follwing:
FSB: 800/533MHz
RAM: 4x DIMM Dual Channel DDR2 400/533 Max 4GB
I'm wondering what FSB/memory speed combination would provide the greatest stability and allow the memory to run in synchrounous mode.  Would it be
FSB 800  DDR 400
FSB 533 and DDR 533?
What would be the best configuration in this case.
It's a P4 LGA 775 motherboard, I'm probably going to be putting a P4 3.2 in it
thanks in advance,

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Agree with all who state that sync ratio is faster (upto 30% in my system, using different configs), so I would suggest using the 800/400 option when not willing to experiment. It's also true that lower cas is faster (5-20% difference). I had my DDR-3200 running at 333, CAS 1,5 (rare option) which was faster than 400, CAS 2.0. Use PC-3200 DDR with CL2.5 (will run on 2.0 easy). The truth is that this question can only be answered in general by pointing to factors that infuence the performance of the CPU/CHIPSET/MEM and their tuning requires some work. (For instance 533Mhz mem on 800Mhz might be faster if its stable at 2.0 when rated 2.5).

My suggestion: Use Sisoft Sandra and measure CPU performance, (CPU) cache performance and memory performance (bandwith troughput and efficiancy). Get it at Guru3d: 

It's the only way to get a definitive anwer!

Blue Rishi  
FSB800/DDR400 most assuredly...T
Match the memory to the motherboard, IF you use a differnet speed memory you will not get the full throughout aviliable. I know I did that stupid human trick, The machine runs great, but not as great as if it had matched the memory speed to the FSB.
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The best would be to use DDR400 memory (aka PC-3200).  This will run in sync with your processor's front side bus and ensure the maximum bandwidth is available.  It is fine to buy faster-rated memory if you can afford it, but you will not gain any performance benefit, it will still run at 400mhz.  You just have more headroom if you want to upgrade later or want to overclock :)

One thing that makes a (small) difference in performance is memory timings.  Not all DDR400 chips are equal.  This is the famous CAS/CL or latency rating.  Lower latency means better performance.  Normal latency for DDR400 is 3.0 ns, usually identified as CL3.  CL2.5 memory costs a tiny bit more but runs faster, which means your system will spend less time 'waiting' for the memory to respond.  One second, divided by 400 million (mhz) equals 2.5 nanoseconds, so CL2.5 is as fast as your bus can communicate anyways, at least without overclocking, so don't be fooled by anyone selling CL2 DDR400 memory it is pointless and probably not true anyways.

If your motherboard gives you the option to run your memory faster than the cpu, don't do it as it will be slower in practice.
If you are going to use the 3.2GHz CPU that runs on the 800 FSB then I would suggest you go for the DDR533 memory. Your FSB determines overall the speed that ALL devices will communicate at. The lower the FSB the lower the overall speed of the system. Memory runs on its own Bus speed, which is indirectly linked to the FSB. The motherboard will automatically adjust the memory speed according to the speed that the memory itself can support and according the to overall FSB of the system.

So, If you but the 3.2GHz FSB800 CPU, then go for the DDR533 Memory / OR DDR400, it doesn't matter both will work and the same goes for if you buy a CPU running at FSB533. Another thing that is often overlooked is "cas latency" Different memory manufacturers offer different "cas latency" 's. Find a reliable memory chip that works at a very low cas latency of at least 2.5

Bottomline: Buy the 3.2GHZ CPU, get the DDR533 RAM with the lowest cas latency. The motherboard will work fine, and it should automatically work out the FSB's itself.
tfjeffAuthor Commented:
yeah, I'm starting to get a feel that I was right in the first place, thanks for confirming it.  Unfortunately, I haven't built the machine yet so I can't benchmark it =).  I'm actually thinking of going with an AMD chip now, and they have a true 400 MHZ FSB, none of this quad pumped crap.
tfjeff, I can only say one thing: smart choice. AMD is almost as fast as P4 (faster in some situations) while offering a much better price/performance ratio. One thing though, for max. performance use a board with Nforce chipset, not VIA (2nd best) and stay away from boards with SIS chipsets. I have good experience with the NF7 from Abit. If you want to spend a little more (as you save some on the proc.) take a look at the AN7:
VERY complete and tweakable/useful for OC-ing. Also, if your budget alows it, consider upgrading to AMD64 (drewl) instead of Athlon XP, it's (way) more expensive than Athlon XP, but it'll give you the fastest system availible (when using a decent mobo). Just ask yourself if a bit more speed is worth the extra money ;-)
Check Tom's hardware for performance difference between P4, Athlon XP and AMD64:

Athlon XP vs. P4 (general/all chips):
P4 3.2 vs AMD Athlon XP 3200+:
P4 (Extreme) vs AMD 64 (FX):
...and many more articles on this site

Best of Luck

Blue Rishi

tfjeffAuthor Commented:
final decision, asus motherboard, athlon xp 3200 proc, 512 MB DDR 400.
Thanks for the input guys.
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