This is a tough question to answer as there are a lot of variables in play, although at first blush 500 users is a significant user base for a single server.
There are many questions you need to think about when trying to configure the right server for your business, such as what applications, and how much will your users use these apps? What is the network throughput needed? What is the disk queue, network queue, CPU queue look like in perfmon when running these apps? Do you expect your clients to access the server two or 200 times a day?
This is truly a complex question which needs a qualified network/server engineer who is familiar with the specific server and client applications they use, along with the frequency of use. I'd recommend you locate a qualified Intel reseller who is familiar with your IT and business workflow. Using this, along with logging and analyzing your current server system performance will give them in needed information to properly size a server system for your company with room to grow in the future.