This option allows you to choose from a range of RAM (a.k.a memory) sizes. Your choices start at one stick or module of 512MB, all the way up to 2GB (2,048MB) of RAM. The options that you see include items such as 2x512, which means there are two modules, each at 512MB of RAM, which is a total of 1024, or 1GB of RAM. Most home users only need one module of 512MB of RAM.
Corsair is the product that is prominently sold due to its high reliability and good pricing. If you wish to use a different brand and/or RAM configuration, please say what you want in the Comments box at the end of the form.
The TwinX memory is RAM that has been specifically paired up to virtually guarantee the best performance and/or compatability between a pair of memory modules.
The memory modules that are rated at 533MHz or higher are only for use in faster Pentium systems. Unless you know you need to choose a 533MHz or 800MHz option, choose the 400MHz option and Vanco Computing will inform you if you can use the higher-speed memory modules.