I have a few comments...
How about getting a full Silicon Graphics / Discreet Inferno setup? Oh, wait, that'll cost about $500,000! 😛
Seriously, though, I would seriously consider getting an Apple G5 and either a 30" LCD or dual 23" LCDs. If he's using MiniDV, DVCAM, DVCPro25, DVCPro50, or DVCPro HD... then an Apple and Final Cut Pro is the way to go. Everything will work out of the box and Mac OS X and the G5 are absolute monsters when it comes to working with the DV25 and DV50 codecs. Converison to MPEG-2 is pretty fast too. Other great add-ons are DVD Studio Pro and Shake. If you need to work with HDCAM, BetaCam, or some old analog format (like Hi8, SVHS, etc) then you'll need to do some analog capture card research. In the Mac world, there are many certified for use with Final Cut Pro.
If he plans on using this Ultimate Editing Machine as his part-time Ultimate Gaming Machine.... then it obviously can't be a Mac. **BUT** I would strongly discourage you make a single PC for both editing and gaming. You don't want problems when you need to do real work. If you need a gaming machine as part of the $10K budget, then please consider building two PCs... one for editing, one for games.
Also... Mac or PC, you should consider spending a few bucks on some external USB2 or FireWire (FW400 or FW800) Hard Drives. We have found these to be much more handy than an internal RAID. Especially after a few months, a few more employees, and a few more machines. It's so much easier to unplug and move around an external 300 GB drive than it is to transfer a file over the network, even with gigabit.
If you need RAID, Mac OS X supports software raid... and there are PCI RAID cards for Mac... or you can install a FibreChannel card an use Apple's excellent xServe RAID box.