You can view Task Manager for CPU usage and there are some good monitoring tools to graph that over time to see if you are maxing the box at high usage parts of the day/night. Memory, the more the better, (if you are 64 bit)..... Database sizing, only you will know what dbs you have in SQL Server and know the growth pattern to know if you will need more space over then next 12 months to 4 years. You can run profiler to capture longer running SQL statements to see if you can add tuning indexes to make it run more efficiently.