There are a lot more factors to consider before making a recommendation on hardware
- How many users?
- Location of users?
- Network infrastructure?
- Peak times?
- How will users be using the server? HTTP service requests? Pulling features into desktop clients, geoprocessing services, cached services, etc etc etc
My answer to this question is that you should undertake a more thorough investigation into capacity planning before making a purchase into hardware. This will minimise risk of overspending or not spending enough for the load your project will have on the hardware.
The XLS tool from Dave Peters is specific to Esri, but will guide you on what kind of load your existing hardware can handle, or in your case, fill in what you expect you need for your project, and it will tell you what hardware specs will handle the load.
As a starting point, I sometimes look at Esri's business partners to see the promotional deals they offer to see what kind of hardware they use. Take a look at the offers for Dell and Esri.
See this answer for a similar answer.