Precision 670 Memory Upgrade

I recently purchased a memory upgrade for a Precision 670 that I have here at my office, and found that the memory I ordered wasn't compatible.  What I purchased was 2 kits of PQI TURBO 2GB (2 x 1GB) 240-Pin DDR2 SDRAM Unbuffered DDR2 533 (PC2 4200) Dual Channel Kit System Memory.  My original configuration included 2 512 sticks that came from the factory. ( this from the service code lookup DUAL IN-LINE MEMORY MODULE, 512, 400M, 32X72, 8, 240, 2RX8). 

 

I know that the memory I bought will not work but why?  Is it the fact that it is unbuffered and non-ECC or is it the speed (PC2-4200 as opposed to the original PC2-3200)?  What specifically is required for memory to be compatible with this system? Registered? ECC? PC2-3200? I am pretty sure it needs to be Dual Channel, but is that correct?

 

Any help would be greatly appreciated.  Thanks.