The bus from a SIMM to the actual memory chips is 32 bits wide. A newer technology, called dual in-line memory module (DIMM), provides a 64-bit bus. For modern Pentium microprocessors that have a 64-bit bus, you must use either DIMMs or pairs of SIMMs.

Memory Packaging 
Information from Hardware Central on SIMMs and DIMMs

SIMMs
An article from The Pc Guide with SIMM information.

Upgrading Your RAM
Make the most of your programs with more memory.

Yahoo!'s memory retailers page
Yahoo!'s directory of memory retailers.