This Lexar memory will run at 800mHz. If your computer can't accept RAM that runs at 800mHz, or if you're running it alongside slower memory, then the Lexar RAM can be downclocked by the computer to run at the slower 667 or 533mHz. It will likely be incompatible with computers using DDR2-400 RAM.
This downclocking feature isn't exclusive to the Lexar memory, though; most RAM is able to downclock to lower speeds when necessary.
Hope that helps!
Re: Hi! I’m considerin
According to Dell's website, the Optiplex GX520 will use 400mHz and 533mHz DDR2 RAM. So, the Lexar memory you posted should work, since it will downlock to the necessary 533mHz.
Now, having said that, note that while your computer can handle a maximum of 4GB memory, Windows XP will likely only be able to use a little more than 3GB anyway, due to limitations in how it was made and how it uses memory. You may be able to save some money by getting a single 2GB stick and a single 1GB stick, and none of that memory will go to waste.
In order to use the full 4GB of memory your computer can handle, you'd need to upgrade to a 64-bit version of Windows, which I'm not sure your processor can handle, and at that point, with the cost of all those upgrades, your money is better spent on a new computer anyway.
tl;dr version: A single 2GB stick should work along the 512MB/1GB memory module you've already got. Buying any more memory is not advised, as the computer won't be able to use it to its full potential.
