Check out my setup in my tagline. I've got a very similar setup. Mine's not in the trunk, though. Mine's going under my drivers seat, with the DVD drive in the dash.
Hard drive brand is primarily personal preference. Most of them are fine. Any of the big names are good: IBM, Seagate, Western Digital, Maxtor, etc.
Mount your hard drive vertically rather than horizontally (like normally in a PC). This will reduce the minor chance of damage to your HDD platters and heads from shock.
Also, when purchasing a HDD look for a model that does NOT have a fluid bearing. This is only important if you live in an area that has freezing/cold weather.
5200 rpm hard drives draw less power and don't heat up as much. 7200 rpm drives give off noticable heat and draw more power. You should be fine with either on the Opus 150 watt supply. I'm using a 5200 rpm drive on the Opus 90 watt.
For memory, get either one 512Mb PC2100 ram stick or one 256Mb. If you go with 256, be prepared to use XPlite to keep WindowsXP from hogging all your ram. I got 1Gb of ram before I realized that it doubles the amount of time it takes my system to hibernate. (more ram to write to disk)
I'm not adding any sound card to my system. The built in sound on the M10k board is good enough for me. I'm not an audiophile. Many people on this forum use the latest USB Audigy unit from Creative Labs.
You won't need to add a video card, unless you plan on playing 3D intensive games. The built in video does very well for windows apps and DVD playback. It even has built in Mpeg decoder hardware.
For the Xbox, you should be able to hook that up to one of the alternate inputs of the Xenarc. I'm not sure (I don't have one) but I think they have a VGA and an RCA input. So you don't need to buy anything for that, other than patch cable and some way to power the Xbox. (don't forget about a patch cord to get the sound from the xbox into your head unit)
To put the PC in the trunk, and the DVD drive up front, you'll want to use either a very long IDE cable (I've got a 10 foot one) or a USB (or FireWire) to IDE converter. I would reccomend FireWire over USB. The 10 foot IDE cable is unsupported and exceeds the IDE specification. So there's no guarantees. Mine works. YMMV.