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What are these PC533, PC667, or PC### etc

What do those means and how will I know what PC### does my old computer have?

Basically, I have Intel Dual Core 2.8ghz and Pentium 4 3.0ghz HT and their motherboards are ASUS and Samsung Polaris.

How can I understand these PC###'s?

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ok thanks to all those answers. anyway, How will I know what "Bus Speed" to use for my Motherboard? I mean, am using a 2 2nd hand motherboard with no manual or chipset driver installer.. and am not sure where to look for supported DDR1 Bus Speed. – Nullstr1ng Aug 26 '10 at 9:43
ok I found something it said "Polaris-30 Socket 478 FSB 800 Motherboard" does it mean "FSB 800" is the motherboard's supported memeory module? It supports up to 800Mhz? – Nullstr1ng Aug 26 '10 at 9:47

Also, if you combine multiple sticks of RAM, they'll run at the slowest rate of all of them. Using 3 PC800 and 1 PC667 sticks, they'll all be forced to run at PC667 speeds.

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It's the type of RAM chips used by your motherboard. If you google the modle number of your motherboard you should be able to find out which you need. Most online stores have Lookup tools too.

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thanks Chris. what about compatibility? – Nullstr1ng Aug 26 '10 at 4:54
If you use slower RAM modules than your motherboard/chipset supports, it will run at that slower rate. If you use faster RAM modules than it supports, it will use them at the max. speed the motherboard supports. If you mix memory modules of different speeds, all of them will run at the same speed of the slowest one. (This is true for modern memory types for PCs, e.g. DDR2, not for all memory types, but I guess thats' what you're talking about.) And of course you need to use the correct memory type for your PC (e.g. DDR, DDR2, DDR3, etc. are different). – JanC Aug 26 '10 at 8:45
YMMV when using memory that doesn't match the specs of your motherboard. In my experience slower modules didn't work in faster motherboards and faster modules work about 50% of the times I have tried in slower motherboards. I've had a motherboard reject a misspec-ed module but accept another of the same vintage. Matching the spec number is the only 100% reliable thing I've found. – Chris Nava Aug 26 '10 at 19:20

These are also called as Bus Speed of RAM. You can the RAMs with up to the what your mother board supports. i.e., if you mother supports PC667, then you can use PC667, PC566, or any speed lesser than that. However, to get better performance, it is suggested that you use max supported speed.

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