Our current server is running Windows 2008 with 2 cpus (Intel(R) Xeon(R) CPU E7- 8837 @ 2.67GHz) with 4gb RAM and runs the database server, IIS, two osTicket installations, and 8 external facing web pages. While we do not have 1 to 1.5k tickets perday (closer to 30-50) we're also running 12 databases, and a bunch of domain we pages. Our cpus usually sit around 40% usage. I would be surprised if you needed much more than what we're using.