REQ: 2gig RAM for DEll 6400, DDR2 SODIMM CL5 PC2-4200/5300

I agree the Crucial checker is a cinch to use and there's no commitment to buy from them. You can always make a note of the memory specs they recommend and then shop around elsewhere. I've found dabs.ie to be quite cheap right now. This 2Gb kit of 5300 modules is €71.33 including delivery: http://www.dabs.ie/productview.aspx?Quicklinx=4B1B&PageMode=1 alternatively you could order two of these 4200 modules for €69.20 including delivery: http://www.dabs.ie/productview.aspx?Quicklinx=3PCQ&CategorySelectedId=11150&NavigationKey=11150%2c43190000%2c4294957152 I'd use Ebay for a lot of things, but not for computer memory, there's too much fake, cheap and unreliable memory chips around, you just don't know what you're getting.

I'm not an expert on memory, although Wikipedia says: "....the CAS Latency is the time (in number of clock cycles) that elapses between the memory controller telling the memory module to access a particular column in the current row and the data from that column being read." and "When selecting a RAM card, the lower the CAS latency (given the same clock speed), the better the system performs." So, when it comes to CAS Latency, the lower the number the better.

If you're adding to memory modules already existing in your system it would be ideal if the new modules matched their CAS Latency. All the memory modules will function at a lower speed if you install new modules with a higher CAS Latency. Unfortunately the reverse is not true, if you install memory modules with a lower CAS than what's already installed, the memory will still work at the higher CAS Latency. I don't think this applies in your case, as your system's maximum memory is 2Gb, so you're free to choose the CAS Latency you what, bearing in mind that lower is better.

As far as I know, the density of the modules doesn't matter, it's only given as a technical spec, for example if you were matching existing modules in your system. I'd suggest you have a look at the Building and Upgrading forum, if you posted this question there you might get a more useful and knowledgeable response.