Mining with HP servers is a terrible waste of electricity. It sounds like you're going to try and put video cards in them for GPU mining. That means, you'd be running the server's many fans, SCSI hard drives, redundant power supplies, and other excessive server-grade hardware unnecessarily.
The goal for mining rigs is to maximize GPU compute output while minimizing power draw. This means sticking many GPUs in a tiny, underpowered motherboard. You want 6990's powered by an AMD Sempron with no extras.
Think of it like sticking a Dodge Viper V12 engine in a Geo Metro; all power, minimal weight. Putting GPUs in an HP server is like sticking a Dodge Viper V12 engine in a 14 car freight train; decent "power", but way too much extra weight.