Hardware Sizing for 5 years

Hello everybody,

In my company we are using NAV 2009 for 2 years now. Today the database has a size of 400GB. We have about 380 Users working on the system currently.
Next year we would like to upgrade to a newer version. For this we would like to buy new more powerful server. At least one new server for the SQL Database and one Server for the Service Tier.

I would like to calculate this for at least 5 years. So we will have a 1000GB database.

I thought about something like this:

SQL:
Intel® Xeon® E5-4600
RAM 1TB
HDD 4TB
Ethernet 2 x Intel QuickPath Interconnect (QPI): 6,4 GT/s; 7,2 GT/s; 8,0 GT/s

ServiceTier:
Intel® Xeon® processor E5 2600 v4
RAM 32gb
HDD 500GB
Ethernet 2 x 10Gb

What do you think about it? Is it too much or too low? Is there a guideline how to calculate such things?

best regards
Max