My preferred configuration for software I write is:

Xeon or i7 based system + 4 or 8 GB RAM + 4 HDD in RAID 10 configuration with integrated linux RAID. If you have budget for more you can get good RAID controller. I use Adaptec 2504. It is cheap and offers enough features.

Finally it all depends on the workload you have to cover with this server.

I also have small installations with much older hardware with RAID 1. But for what they do they are good enough.