This gives 16Bit to hold the information needed for ECC for every 128Bit of data stored. The capacitors C3, C4 are decoupling condensators for higher frequencies. C25 is a filter condensator for lower frequencies. In addition to this you will commonly find a decoupling capacitor for each DRAM-Chip, either under or next to the chip. Likewise a . represents a open connection.
Some systems check for the actual SIMM speed installed.
Installing a faster SIMM than specified may not work even though technically it would. Higher-end computers use DIMMs, which are usually 168 pin. The jedec standard describes basically 9 different industry standard DIMM's. So a mixup can not damage neither the system nor the DIMM, as it is not possible to insert the wrong DIMM's into the slots. Compaq uses the newer 3.3V technologie, and the DIMM's are double buffered.