well, different 'types' of RAM have different qualities and ultimately, sizes.
Standard SDRAM (hardly used anymore) single speed with 168 pins.
DDR SDRAM (again, hardly used) is twice as fast (DDR means double data rate) and has 240 pins.
DDR2 and DDR3 SDRAM are 8x and 16x speed (i think) and have 240 pins per DIMM.
so, SDRAM wont fit in a DDR/2/3 slot, and DDR2/3 wont fit in a DDR slot. If the motherboard has DDR3 slots, you will need DDR3 memory.
it's all rather complex really, i'd advise really reading up on this stuff before undergoing an upgrade of this magnetude.