>I know that it is the 4200 and the 5300, but that seems to be the only difference. The price is the same. I think they are both compatable with my laptop, so why get one over the other and why no price difference?
>
>

>4GB kit (2GBx2)
>
>CT533345    DDR2 PC2-4200 • CL=4 • Unbuffered • NON-ECC • DDR2-533 • 1.8V • 256Meg x 64
>
> 4GB kit (2GBx2)
>
>CT533355    DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 256Meg x 64
>

The 667MHz part gives you higher memory bandwidth (more GB/sec transfer) if your laptop mainboard supports it. If your laptop doesn't support it, you won't see any difference.

Whichever one you go for, make sure it actually is compatible. Laptop memory is usually non-standard, i.e. uses a different size/shape circuit board than standard RAM for ATX-style motherboards. These circuit boards are sometimes manufacturer-specific, especially for laptops that are aimed at being small and light.

Laptops also sometimes have nasty surprises such as having one relatively low capacity stick soldered to the mainboard, and only 1 other slot available for expansion. If you can, open up your laptop and check if the existing RAM is removable and what your expansion capability is.

You might also want to make sure that your laptop can support 4GB RAM, some might only support 2GB max.

Yet another thing to be careful of these days is that some RAM is dual-channel (which effectively doubles memory bandwidth). This type of RAM is sold in pairs of RAM sticks and will usually be sold as a dual-channel "kit". Dual-channel RAM might be worthwhile if your mainboard supports it but it is a bit more expensive.

The reason the 4200 and 5300 parts are the same price is that the 4200 is getting old, not made as much these days so it's a little harder to find. Even though the 5300 is higher performance, it's more widely available so at this time the price difference is little or none. What you find over time is that the price for old RAM is actually much higher than for newer, higher performance RAM. It can be more expensive to upgrade RAM in an old mobo than to buy a new mobo with the equivalent amount of new RAM.