Question about RAM for my HP Pavilion 576X

[Relztrah]

According to Crucial.com, "Each memory slot can hold DDR PC3200, DDR PC2700 with a maximum of 512MB per slot." The one stick of 512MB that is currently installed is PC2100...again, according to the system check from Crucial.com.

So what does the number following PC signify? I used to think that was the speed, but I believe the stick I have in there is 266 Mhz. from the label on it. I want to install another 512MB of DDR RAM and I'm pretty sure I can mix and match RAM speed although I understand that it will only run as fast as the slowest stick. But I also want to make sure it's compatible with my system. I'm not sure it matters, but I use UNBUFFERED, NON-ECC RAM modules. Do I need to concern myself with the 2100, 2700, 3200 or whatever, or just make sure it's 184 pin DDR RAM?

[jlreich]

So what does the number following PC signify? I used to think that was the speed, but I believe the stick I have in there is 266 Mhz. from the label on it.
It is the effective speed of the ram. 266x8=2128 - or rounded off = PC2100.

I'm pretty sure I can mix and match RAM speed although I understand that it will only run as fast as the slowest stick.
Correct.

I'm not sure it matters, but I use UNBUFFERED, NON-ECC RAM modules.
That is what you want to use. Buffered and ECC ram is generally only used for servers and is more expensive.

Do I need to concern myself with the 2100, 2700, 3200 or whatever, or just make sure it's 184 pin DDR RAM?
Definitely don't get slower ram than what you have in there, you will bring down the speed of existing ram. I usually recommend getting DDR400 (pc3200) as it will work just fine, cost the same, and more likely to be able to be sold or used in another system later on.