I think that the 168 pin are PC 100/133 Dimm (older)
The 184 pin are DDR dimm (newer)
In several speeds, it will depend on your motherboard and its FSB speed as to what ones you need.
more mem info click here
Some motherboards have slots for both types but I don’t think they can be mixed.
If you are adding memory you will need the same as you have, if you are replacing memory then the 184 pin DDR is better.
More info on motherboard please