I was wondering if 333 mhz memory would function in a desktop that normally used 266 mhz? Are there differences in footprint or electrical or pin count which would prevent this? I am not trying to speed up a computer, just consolidate my memory types between two machines.
There are different "footprints" as you say for different memory types, typically it is defined by the "DDR" type...DDR, DDR2, DDR3, and the newest, DDR4. Within each type there are different speed ranges, such as the PC2100 and PC2700 you mentioned.
Motherboards will support a certain type of memory (DDR2, DDR3, etc) and will also support up to a certain maximum speed and amont of memory. To use your example, if your motherboard will support PC2700 memory, it will usually also work with PC2100...memory speed for the supported type is almost always backwards compatible to slower speeds. Unless the fastest speed supported is too expensive, there is no reason to put slower memory in it.
Also, don't mix different speeds of memory. If you want PC2700 in it, then use all PC2700 sticks, don't put a stick of 2100 and a stick of 2700 in it.
Consult your manufacturers site, or user manual to find out the type and the fastest speed and maximum amount of memory your motherboard will support.