What is the difference between DDR1 and DDR2 ram?

DDR2 will not fit in DDR1 notch in the first place (DDR2 will physically not insert into the socket of DDR1).

The difference between DDr1 and DDR2 is, On the physical side, DDR has a 184-pin interface and DDR2 has 240-pin and they also differ in the voltage they use, DDR1 uses 2.5 Volts DDR2 uses 1.8 Volts. As you have mentioned speed also differs DDR1 has 200 MHz, 266 MHz, 333 MHz, 400 MHz and DDR2 has 400 MHz, 533 MHz, 667 MHz, 800 MHz.