Chaos, you're right about Xeon 5540 only supporting 1066MHz memory. You can still use a 1333MHz memory, but it will run at 1066MHz. If you're using an Intel server board, please make sure you're using tested memory. There are many other factors that affects memory performance, like SR/DR/QR, number of DIMMs, etc. You can use the Memory Configuration tool to determine your memory configuration.
Yes, you should be able to populate your system with 1333 MHz memory with the E5540 CPU, but the max speed the memory will actually operate is 1066 MHz. Then if you choose to populate with a X5550 or above at a later time, then the system memory should then operate at 1333 MHz.
Don't forget to check what memory types your vendor supports for your particular systems.
Hope this helps...Ed
Thanks for the response guys. It turns out that this particular processor requires Registered ECC RAM. The memory that we received was unbuffered, so that would be the big show stopper. Guess we'll be sending back for the registered RAM type.
Chaos, the CPU DOES support both Registered and Unbuffered DIMM, but you can't mix RDIMM and UDIMM in the same system. RDIMM is only required when you need to use a DIMM > 4GB size, because Registered DIMM can provide larger capacity. Which server board are you using? You may want to check if there is any special requirements for your server board...