you can't just pick any old memory and plug it in if it "fits"
You can smoke your memory or your motherboard by plugging in "any old memory that fits".
If you have a major manufacturer: Dell, Compaq, HP, Gateway, Emachines, etc
- Order the memory through their web site (Dell). You will need to know the Express Service Code of your PC.
- Order the memory through HP (or other mfr) Tech support. you will need to know the model of your PC.
- order replacement for ALL your memory (suggest a max-out if you can afford it and your OS support it) through a 3rd party memory company such as corsair.com (cheaper) or memorysuppliers.com (even cheaper). look up your computer model and determine the proper memory type and how much to buy (should say how many slots your machine has - sometimes they are wrong about the number of slots, so double-check the number of slots).
32-bit OS supports maximum 4GB memory. 64-bit has a HUGE limit, but most desktop motherboards are very limited compared to servers and either handle 16GB, 24GB, or today's 8GB max. You can tell which type of OS you have by doing Start, right-click on My Computer, and looking at the information presented in the dialog box.
if you do NOT have a major manufacturer
- you must look at the manual for your PC (the cd/dvd that came with your motherboard contains the manual, or look it up online), and determine the proper type of memory for your PC.
- other alternative is to look up your motherboard model in a memory supplier's web site and see what kind of memory it takes, an then order it.
Don't Mix Memory types, memory manufacturers, or timings!
Even with PC-133, there are CL2 and CL3 memory timings. And you can't go mixing those on the same motherboard or you are looking for trouble (crashes, file corruption, apps don't work, etc)!
When you go to buy Dell memory from 3rd party manufacturers like corsair.com or memorysuppliers.com, you don't know what you are getting. Corsair etc. tells you what the timings are, but Dell doesn't. so if you are going to buy memory from a 3rd party supplier, replace ALL the memory, unless you know from stickers or previous purchase that the memory is in fact the proper timing.
motherboard "Approved memory"
Motherboard manufaturers can't test with every kind of memory that is out there. that would cost too much money. and at the time of manufacture, there are fewer quantities of memory available for the motherboard.
So there is an "Approved" list you can go by if you like that "safe" feeling. If cost or something else is a concern or you want more memory installed than what they list (such is the case with the unusual 24GB motherboards), you will probably have to go with something that is not on the list.