Interesting.

I have a T7200 2.0Ghz cpu in a D630 with 3GB of DDR2 800Mhz ram memory which I have been using for weeks with no problems. It says on the memory modules 800Mhz, and I noticed that the old ones said 667Mhz. Is this a typo of some sort between manufacturers? As well the model number of the chips were different. The 667Mhz read as PC2 5300S and I believe the ones that say 800Mhz on the manufacturer's sticker is listed on the manufacturer's sticker on the memory modules as PC2 6400S.

My point is, are you sure that i can not use 667Mhz memory with that T9600 2.8G cpu chip? The original memory I took out shows that they are 667Mhz, but the ones I recently put in it say 800Mhz, so please excuse me if I get confused. It came from a box of spare sparts and the computer functions as well as when it had the PC 5300 memory.

No - worst case is you get four beeps at powerup and have to change the RAM.  You won't harm the system or the memory.

The RAM needs to be one step behind the CPU bus for these systems - if you install a CPU that runs an 800 MHz bus, 667 RAM is fine.  Likewise for a 1066 MHz CPU such as the one you want to install - the RAM needs to be one step lower (800 MHz).  You can try 667 with a 1066 CPU but I suspect you'll get a memory error when you try to boot the system.

Is that the worst that could happen?

I couldn't fry these memory modules because they are the wrong ones to use with the cpu?

Instead of using DDR2 800mhz memory for a Core Duo 2.8Ghz cpu, can I use a DDR3 1066 instead?

Are they compatible?

What model DELL laptops can handle DDR3 memory? I can see that DDR3 memory is half the price of DDR2.