It all depends on a couple of things:
1. How many people will be activily using Service Desk.
2. How many tickets are created on a weekly basis.
3. How many of those created tickets will stay open.
But as far as minimum requirements I would say:
SQL on a seperate server (no VM)
Service Desk server:
8 cores, 16 Gb of memory.
For testing only you can get away with 4 cores and 8 GB.