It was agreed to start with a z400... the workstation tests were actually developed on this machine, hence why it has a Gen 1 board. Mayhap we'll look at a z800 later.
Most video editors use a RAID array for their video files unless they are just editing DV. HD footage shot on a decent quality camera would quickly overwhelm your proposed 500GB array- the raw P2 footage alone for my film project was 350 GB, and that's before the scratch files generated for color correction and transition renders- and the P2 footage is pretty strongly compressed for semi-pro camera footage. (40 Mb/s) If you're editing uncompressed 1080 HD, you're dealing with 124 MB per second of video. With a larger array, a system like this could be set up for editing uncompressed HD with the simple addition of a BlackMagic Decklink in one of its HD flavors, or the HD version of the Avid Mojo if you prefer to work in something like Media Composer. With the transition of TV to HD, more and more projects are being finished for HD in an attempt to future-proof them- which has actually been going on for about five years before the transition. If you want to see something truly horrible, look at the data rates required for 2k or 4k playback...
HP doesn't sell their machine configured with any kind of professional audio interfaces. They submit their machines for certification by Avid, and the 'full' Pro Tools (Pro Tools HD) comes from Avid on a z800.The audio interface choice for the system was either the Creative card, or the onboard RealTek codec, because that's what HP had and could configure a system with.
You'll find a surprising number (like, most) audio guys including musicians these days working with a mix of software and hardware, and the mixing is done on a computer, not on external hardware. There are audio interfaces with two, four, six, eight and more inputs... my personal music workstation has eight analog inputs, plus ADAT and S/PDIF in. (Note, I need more like twice that in inputs.) You need all these input to capture multiple instruments on completely independent tracks simultaneously , so that they can be given separate effects and the mix can be adjusted on each track. (That's right, you can capture all those tracks at the same time.) Some of these audio interfaces are an internal card , some are an internal card plus an external breakout box (M-Audio Delta series, MOTU 2408, etc), and most are USB or firewire external (Presonus, TASCAM, Echo, Digidesign, Focusrite, et al) boxes with all the hardware located within it. In addition, many have PCI or PCIe DSP accelerator cards from UAD or TC electronic for effects processing and synthesis.