Your PCs main memory is called by a number of names: RAM, DRAM,SRDRAM, DDR SDRAM. Don't be confused by these names. They are all types of RAM(Random Access Memory).
PCs first used DRAM (Dynamic Random Access Memory) for main memory. These wereimproved and became SRDAM (Synchrounous DRAM). The latest version is a DDRSDRAM which is a (Double Data Rate SRDAM). It runs twice as fast as SDRAM.
Main memory is used to store programs and data electronically when your PC isrunning. It communicates with the processor over the front side bus. Memorycapacity is measured in megabytes Mb (millions of bytes).
A big memory allows your PC to run multiple programs with out slowing down andwith much less of a chance of crashing. Your memory also needs to be fast andmanufacturers offer it in a number of speed grades.
Multiple memory chips are packaged together on a small circuit board. Thisassembly is called a DIMM (Dual Inline Memory Module). DIMMs plug into themotherboard. SDRAM and DDR SDRAM memory chips are still found in DIMMS, but theSDRAM chip have pretty much been replaced by the much faster DDR SDRAM chips.
Recommendation:
Get at least 128Mb of DDR SDRAM at a speed grade of PC2100. Make sure your PCmotherboard has sockets that accept DDR DIMMS and a memory controller capableof running PC2100 memory. Any new PC motherboard should be able to supportthese requirements.
SDRAM DIMMs are still available in the after market. They have 168 pins andrequire a socket different than that used by the 184 pin DDR DIMM.
Some motherboards have both types of sockets that allow you to use either typeof DIMM. You can't use both types of DIMM together. It's all one or all theother.