When I got the Dell R510 it came with two Intel Xeon L5520s. I wanted to go even more lower powered (the plan is to have it running 24/7), so I got two L5630s. While the L5520 has a TDP of 60 Watts, the L5630 is 40 Watts, so quite an improvement.
I initially installed only one CPU, wanting to give that a try. I wasn't really enjoying the performance though, so I ended up deciding to add the second L5630. This would also let me add more RAM. The R510 came with only 8GB RAM, which depending on your needs might be enough, but it certainly was pushing it for what I want to do with the server. So I decided to, for the time being anyway, replace the 8GB (2x4GB) of registered PC3-10500R RAM with 8 4GB PC3-12800U (non-registered) RAM, giving the server a total of 32GB.
I also took the opportunity to install new network cables I got, as the ones I had been using up until that point were actually too short to fit comfortably. While the new cables are CAT.7, I don't need them to be, and only picked them because they were properly shielded and seemingly well built, as-well as the right length.
After these upgrades FreeNAS is noticeably snappier (or, less slow). I have yet to finish setting it up fully as there's a few more things I plan on setting up in jails, but so far I am very pleased with the R510. With both L5630s installed and all 12 drive bays filled with spinning disks, power consumption seems to hover around 200 watts. This went down with more recent upgrades that reduced the drive count to 8, with power consumption hovering around 165 watts. More on those updates in another post.