Most RAM will be non-ECC and that's what you want.
It's nearly always mentioned in the specifications at the seller's site, whether that is Newegg, Amazon, or Crucial.
You could buy directly from Crucial if you want. Or get the Crucial part number and locate it at somewhere like Newegg.
Or buy another good brand like Kingston, Corsair, or Gskill.
You should also probably go to the laptop manufacturer's web site and locate your laptop. There should be a specification sheet for that laptop that will tell you the maximum memory it can take and what the specs must be. Pay attention to that.
The Crucial configurator is highly accurate, but not perfect.
The main things to pay attention to are the voltage, ECC or non-ECC, buffered or unbuffered, type (DDR 2, DDR 3, etc), speed (PC2 5300 for instance). You also need to make sure you are getting the right DIMM specification---DDR2 for a laptop is typically 200 pin, as far as I know.
Here's a bunch of DDR2 200 pin DIMMs at Newegg. All of it 1.8 volt, but varying speeds and latencies.
Computer Hardware, Memory, Laptop Memory, New, DDR2 400 (PC2 3200), DDR2 533 (PC2 4200), DDR2 667 (PC2 5200), DDR2 667 (PC2 5300), DDR2 800 (PC2 6400... - Newegg.com
Are you
replacing your existing RAM? Or
keeping it and adding more? I assume you just have 2 RAM slots? If you have 2 slots and are replacing all existing RAM, you'd need to buy a set of 2 sticks sold as a single unit--either two sticks of 2 GB each (4 GB total) or 2 sticks of 4 GB each (8 GB total), whatever you want.
If you have Windows 32 bit and have no intentions of moving to 64 bit, there's no reason whatsoever to get more than 4 GB of RAM, total. So, you'd get a set of two 2 GB sticks, for a total of 4 GB.
Here's the Newegg list of 4 GB DDR2 laptop kits, non-ECC, 1.8 volt, 200 pin, PC2-5300:
Computer Hardware, Memory, Laptop Memory, New, DDR2 667 (PC2 5300), 1.8V, 200-Pin DDR2 SO-DIMM, 4GB (2 x 2GB) - Newegg.com
I'd probably get one of the lower priced Mushkins or the $45.39 Crucial; no need to spend more.
