I'm using this CPU with a Supermicro X9SRA. It's ATX form factor (probably won't work with your HP case/power supply) and supports registered (but I'm using unbuffered) ECC DDR3-1600. You can also upgrade to a v2 CPU.
I've had it for 3 years and it's still going strong. The only down side is it's got only 2 SATA3 ports (plus 4 SATA2 and I think some SAS2). I did note some negative reviews on Newegg, after I bought mine. Maybe there was a bad batch, at some point.