"enough" is relative to what you want to do. If this is a development environment and you're trying to test functionality then you don't need a lot of horsepower. On the other hand if you're load testing and need to simulate production strain you will want something that matches what you're got in production. You should also consider what kind of redundancy your system needs. If this is "just development", you don't care if the box loses a power supply, and you don't need massive horsepower you could consider going with something like a quad core PowerEdge 860 with 4 GB RAM, no RAID, and get a next business day hardware support plan.
A lot of people nowadays are buying higher end machines and going the virtualization route. A PowerEdge 2950 with SAS drives, a RAID controller, and 16 GB of RAM can be virtualized into 3 or 4 machines. Not only do you get redundancy but you can get more bang for the buck by stretching your hardware out.
* Note I have no affiliation with Dell. We use them at work so I am just throwing them out there as a point of reference.
Kendal Van Dykehttp://kendalvandyke.blogspot.com/