A IIcx is kind of old for a Jackhammer. I would probably go with OS 6.08, put in an old Supermac or Rasterops high end card (you know the ones you rarely see sold from the very early 90's full of chips), an old 50 pin SCSI card like a Micronet Nuport II, and to finish it off a rare specialty card like an early Audiomedia I or a Spigot.

One of my IIcx currently has a hardware SCSI card that mirrors the main HD and switches over incase of a fault. Have to finish setting that up.

I prefer cutting edge cards in quadras where the CPU complement the added speed of a good video card, scsi card, and maybe a DSP card.

The older 030's are fun to play with the earlier rare oddball cards.