Difference between DDR1 and DDR2 type rams, and a little bit about video cards

I recently just bought DDR2 ram for my computer. I checked my mother board just to make sure that it would work before I bought it. It clearly says DDR ram. I thought well DDR2 is just newer. I bought it. It didnt fit. There is a clear differenece between DDR1 and ddr2. Make sure you have the right slot for your ram.

I also bought a video card for my computer. the slot said AGP 4X. I didn't understand what that meant. I actully called the manufacturer for the meaning. I was looking to buy a 8X agp card. He said that 4x is what the power supply can hold in the motherboard. He said I could buy an 8x ag for it but it will not work at full capicity. So I stuck with the 4x video card. I am hoping for a new computer for christmas!