It's going to likely be a freenas box with raidz.
If that is the case then:
- Skimp on the CPU. An Atom might be a bit slow, but anything else current will be more than enough. It's not like you are going to run much else on FreeNAS.
- Get RAM, lots of it. ZFS loves RAM and is not graceful if it runs out. 16 GB is plenty though unless you use de-duplication.
- Get ECC RAM if you can and your board supports it.
- Get a GOOD UPS.
- Go lower on speed or capacity if you need to, not on quality. ZFS is not happy with substandard parts.