the ram, the processor and the storage .
Ahmed, this topic has been discussed in various flavors more than anything, ideally you should have looked into forums before posting yet another similar question of yours as you would have already found wise replies.
Anyway, regarding user base of 2000, you can register a million users on minimal hardware without issues, where it regards to concurrency, this is where your hardware screams and you as well mentioned chatting (absolutely overkill, brings powerful servers down to their knees) and quizzes for 500 concurrent, then expect no less than a dual socket quad system minimum with combination of SSD drives serving database on RAID and regarding RAM, expect it to be no less than 32GB if you're going on one server setup, else database server as independent hardware is always recommended.
Overall space depends upon, what type of files you're uploading. Video files can take more space than just text chats and quizzes as they are just text based filling up database, and you'll hardly hit 100G in a years time.
I always ask - and rarely get answered - how did you arrive at your figure of 500 concurrent users?