RAM or Random Access Memory is the main memory of your system, where apps have to be stored before being executed. So it is most important for your RAM to be fast enough that it does not become the bottleneck in your system.

There are two types of RAM presently available. They are EDO & SDRAM. Though you may still find FPM(Fast Page Mode) RAMs installed on some PCs, no manufacturer is producing them now nor is its performance anything special to write home about.

EDO stands for Extended Data Output. It has a typical access time of 60-70ns, though you can get those with 50ns accesses. All the EDOs produced support only 66Mhz FSB. SDRAM is the acronym for Synchronous Dynamic Random Access Memory. It access time is in the region of 8-10ns, with 100Mhz SDRAMs also available.
Both EDO & SDRAM are available in both standard & ECC forms. ECC forms are typically used for server applications where reliability is critical.

One thing though. If you are buying SDRAM go in for those certified as PC100 SDRAMs. This ensures compatibility with a majority of 100Mhz FSB mainboards.

Which is best?
No complications here. Just go in for SDRAM- 100Mhz only if you have a 100Mhz FSB mainboard; 100Mhz even if you have a 66Mhz ordinary motherboard.

Next: Video Cards