DDR and or DDR2?

Does anyone know the difference between the 2? Could I put a DDR memory sticks in my PC that now has DDR2 memory sticks?

They have different bus speeds, different pin densities, and different foot prints. DDR2 is not backward compatible to DDR.

Wikipedia:

Backwards compatibility:

DDR2 DIMMs are not backwards compatible with DDR DIMMs. The notch on DDR2 DIMMs is in a different position than DDR DIMMs, and the pin density is slightly higher than DDR DIMMs. DDR2 is a 240-pin module, DDR is a 184-pin module.

Thank you both very much! I figured so since I was shoving it in and it wouldn't fit LOL! So I need DDR2 and does the number matter or can I just buy any DDR2 memory and it work? Can you tell I know squat about PCs "sigh"

You can mix and match different speeds to some extent, but the slowest module you can use is determined by your motherboard. You're best off matching the one you have now, as the memory will only run as fast as the slowest installed module.

Thank you Zaphod, off shopping again LOL! One more question.. how can I tell how much memory my PC will support?

The motherboard determines the maximum amount of memory. So you'll need to try and find out what type of motherboard you have.

The motherboard model might be displayed in your BIOS setup. To access the BIOS, try pressing the 'Del' key while your computer starts up. If this doesn't work, read the boot up message and look for a message like - "To enter setup press <blah>".

WOW Thank you so much 2k