In my experience, if you want to use that HP case then you're gonna need to jerry-rig up some sort of adaptor, as the front-panel headers on their boards are not of a standard layout.
HP tend to use a single block layout which means that trying to use their cases with any generic hardware is a nightmare. I've got around this issue in the past by creating an adaptor board, that hooks up to the block connector on the case using a small piece of prototyping board and a set of header pins, but it's not for the feint-hearted.
You can usually use their main-boards in a generic case, but that's not the issue here.
To do the mod, you need...
Prototyping board (Veroboard)
Soldering equipment
Multimeter
A scrap case
Header pins
Hot glue gun & glue
Exacto-Knife
You can get the Veroboard & Header pins from any electronics shop e.g Radio Shack or Maplin Electronics. Any scrap PC case will do, all you need from that is the front panel header connectors that you will solder to the Veroboard so that you can hook them up to your new board.
Cut off a section of Veroboard with the exacto-knife roughly the size of the block connector on the HP case. Make sure you do this so the tracks on the underside go from top to bottom when orientated horizontally...
[fixed]
+---------------------+
| ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ |
| | | | | | | | | | | |
| ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ |
+---------------------+
[/fixed]
You will need at least 6 rows by however many columns there are on the block connector. Next thing to do is to solder the header pins to the veroboard. Once you've done this, cut the tracks on the board between the two rows of pins with the exacto knife so that the meter shows "open" between the pair of pins in each row. THIS IS IMPORTANT, AS DAMAGE WILL RESULT IF THESE TRACKS ARE NOT CUT PROPERLY.
Next, Lop the front panel headers off the scrap case giving you as much of the cable as you need and bare off the ends. You will be soldering these cables to the outside edges of the veroboard header you've just made. Again, it's important to take your time with this bit as you'll need to read the pinouts on your original HP board to try and figure out what pin does what. The other option is to carefully remove the front of the HP case and follow the wires up from the block connector. Do whichever is easiest. Sometimes the original mobo isn't marked, in which case, the only option is to trace the cables up the front of the case. The exact pinout varies from board to board, but the theory stays true.
What you should have at the end of all this is a piece of PCB that has a load of pins in the centre two rows. Your HP's block connector connects to these, and wires coming out top and bottom, terminating in the header hookups from the scrap case. These will hook up to your new board.
Check everything over with the meter and make sure that all the connections are good and nothing is shorting out where it shouldn't be. Once you've done this and are satisfied that nothing is shorting out and everything is isolated, use the glue gun to encase the PCB part in hot glue. Make sure you leave the pins clear. This will provide insulation and prevent shorts when you place it inside the PC chassis. Once you've done this, plug your block connector on the pins in the veroboard and the corresponding free headers on the other end into your new board.
Good to go!!
I volunteer at a local PC re-use & recycling project and have jerry-rigged a number of PC mobos with custom FP headers in this way. Likely candidates for jerry-rigging are all HP, most Packard Bells and quite a few, if not all Dells too.