The confusing part (at least for me) is that the new 54XX Xeons are cheaper than the similar, older 53XX Xeons. For example:

a. Intel Xeon E5335 Clovertown 2.0GHz LGA 771 80W Quad-Core 2U Passive Processor Model BX80563E5335P – Retail $349.  

b. Intel Xeon E5405 2.0 GHz 12M L2 Cache 1333MHz FSB LGA771 Active Quad-Core Processor $230

These processors have the same 2.0Ghz clock speed, same 1333Mhz front side bus, but the 5405 has 50% more cache than the 5335.  So it’s better and $120 cheaper.  So that begs the question:  Can I use the 54XX Xeons in a dual socket 771 system that supports 53XXs?

Suprisingly the answer is YES, mostly.  The Intel 5000 chipset motherboards claim to support most Xeons from 51XX to 54XX.  You would hope that 3rd party Intel 5000 chipset motherboards would behave the same.   This article  discusses swapping 53XX and 54XX CPUs out in the same motherboard and doing benchmarks. So its definately doable.

There are a few exceptions.  The very fast and expensive E5472 has a 1600 MHz FSB that is too quick for older motherboards that max at 1333 MHz.    The Dell Precision 490 that I like so much has some versions that don’t support any quad-core processors at all. Obviously that old version isn’t going to work with the 5400s.  The newer Dell precision 490′s that support 53XX’s should work fine with 54XXs.

For compatibility with older servers and for exact replacement purposes you will see 53XX Xeons around for some time. But if you have a capable motherboard, I would definately recommend saving some money and using a Harpertown Xeons. Good Luck!