Processor/CPU. You are free to choose the amount of cores and processors in a server. Note that higher amount of cores means better performance. Any multi-threaded application benefits from more cores, including web servers and database servers. Also look at clock speed (MHz) and remember that a server with more cores (4 or 8) and less MHz will perform better that the one with fewer cores (2 or 4) and higher MHz. Of course, you should also consider the number of users.
RAM. The better CPU is, the more RAM options are available. Just in case, order a server that allows adding RAM when needed. Luckily, the number of available RAMs for double processors is great! See what the size of your server is when selecting RAM. Servers of present generation use DDR4 technology, it is affordable and is considered to be a standard today.
Hard drives. RAID set-up is the most optimal for hard drives today, because it prevents failures successfully. With the help of this technology, this system will be operable with minor performance loss until the hard drive is restored. SAS and SSD drives are considered to be the strongest ones today.