Xeon 3400 series is based based on 1156 socket, so in essence is last gen CPUs, they will consume a little more power or be a little less capable, not a big deal on a server though, depending on what you do. C202/C204/C206 are the new intel server chipsets, so they take the new Sandy Bridge Xeon E3 cpus, they should be more powerful / use less power. That said everything is releative, not only the CPU/Chipset will influence the power consumption, there are a lot of other factors that you should account, like hdds, hba, PSU, etc. My server currently is idling at 105W with 11 hdds (still have to install 3 more), on my old Atom 510 i used to idle around 40W with 4hhds in. Btw there is a very low TDP Sandy Bridge Xeon, Intel® Xeon® Processor E3-1220L (3M Cache, 2.20 GHz)
, but its expensive, and hard to get a hold, but lower TDP than celerons.
Depending on what you planning on doing you can also go with atom, there are server grade mobos like SuperMicro Atom
, although idk if supermicro is as efficient as intel on their mobos. I also seen people build around AMD E-350
as with intel sandy bridge celeron cpus like G530
, really depends on what you need or planning on doing with the server.