Um, if I'm reading this right, your E4300 is a Dell Latitude? Should be running much faster ram than that! I checked specs on Dell's site, yeah, you should be using "DDR3", which is the current technology type, "PC3-10600", which is the speed class (my lingo). PC3-10600 operates at a speed frequency of 1333 MHz. What you were referencing was the DDR2 technology, PC2-6400 speed class, which operates at 800MHz. And the 667 and 533 numbers are also references to speed. Hope I'm not breaking any rules here, but here's a quote from Crucial, FAQ:
"Let's take a look at PC2-4200 (DDR2-533) to break it down simply.
PC2-4200 (DDR2-533) refers to the bandwidth of the memory. A PC2-4200 module has the bandwidth of 4.2 GB/sec; therefore, it is referred to as PC2-4200.
DDR2-533 refers to the effective front-side bus speed of your system. While your DDR2 system or motherboard may operate a 266MHz front-side bus, its effective front-side bus speed is 533MHz because DDR2 effectively doubles the amount of data transferred per cycle that a non-DDR2 system would.
The same holds true for PC2-5300 (DDR2-667), which has a bandwidth of 5.3GB/sec and is designed for use in systems and motherboards that require a 333MHz front-side bus, with an effective front-side bus speed of 667MHz. And the same holds true for PC2-6400 (DDR2-800), which has a bandwidth of 6.4GB/sec and is designed for use in systems and motherboards that require a 400MHz front-side bus, with an effective front-side bus speed of 800MHz.
Though DDR2 memory was designed to be backward-compatible (meaning you can use PC2-5300 module in a computer designed to use PC2-4200, or vice-versa), ..."
I recommend getting the upgrade advise either straight from Dell, or from a name-brand memory company. I find the latter to be more informative and easier to understand when I have questions. Hope this helps in some way.