I have a Think Server looking at upgrading it

https://www.cpubenchmark.net/socketType.... (Click on the socket type to ensure the page shows the right CPU's.)

As you can see on the linked page, many of the highest performing CPUs for your socket are pretty pricey. You might find a good deal though on eBay, especially if you are looking to upgrade a personal computer as, many Engineering Samples are sold on eBay as well (grey market items, just not illegal and, typically at about a third of the retail price. (Most will have a base clock @ 100MHz lower than the market released version. Just be sure they reach the same turbo clock speed before you buy an ES version.) You will also want to make sure your motherboard supports the v4 (Broadwell) CPU's if you think you might want to upgrade to one; it might be possible to flash of your BIOS, or the chipset may not support the next gen CPU's (from the 22nm Haswell) E-3-1225 v3 you currently have. (Check with your motherboard maker's website for v4 CPU compatibility before you buy.)

Another thought... The E3-1225 v3 is not a slouch in single thread operations. It may be that you need to just upgrade your hard drive. If you have not invested in a Solid State Drive yet, I can assure you, you have little hope of much improving your system's performance with a CPU upgrade. Many people will invest in an SSD for their older desktops for much faster boot times and very noticeable performance gains for every day applications - like loading/running their browser or office applications. The biggest drawback on your current CPU is that it is a quad core without hyper-threading; (where each hyper-thread capable CPU's physical core has an additional logical core which technically doubles the number of simultaneous operations the processor is capable of running.) That said, if you do not run many applications at the same time, you're not seeing slow performance because of your CPU. Some applications are multi-thread capable but, most common applications only use one or two cores.