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 were improved and became SRDAM (Synchrounous DRAM). The latest version is a DDR SDRAM 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 is running. It communicates with the processor over the front side bus. Memory capacity is measured in megabytes Mb (millions of bytes).
A big memory allows your PC to run multiple programs with out slowing down and with much less of a chance of crashing. Your memory also needs to be fast and manufacturers offer it in a number of speed grades.
Multiple memory chips are packaged together on a small circuit board. This assembly is called a DIMM (Dual Inline Memory Module). DIMMs plug into the motherboard. SDRAM and DDR SDRAM memory chips are still found in DIMMS, but the SDRAM chip have pretty much been replaced by the much faster DDR SDRAM chips.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 PC motherboard has sockets that accept DDR DIMMS and a memory controller capable of running PC2100 memory. Any new PC motherboard should be able to support these requirements.
SDRAM DIMMs are still available in the after market. They have 168 pins and require 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 type of DIMM. You can't use both types of DIMM together. It's all one or all the other.