Get a fast Core 2 Duo as they are excellent at media encoding and 3d rendering work. If he really wants to push the boat out, get a quad core, but that's big bucks.
Need a fast hard drive, but large capacity. Even better, one OS/scratch drive and one huge drive for the media.
It used to be that you needed loads of memory for DV work, but not really an issue any more. For heavy rendering I think a couple of gigs will be fine (not totally sure on rendering memory requirements).
Any mid range 3d card will probably be plenty. Most common cards now will let you set up scenes very easily. You need massive scenes before they chug, and by that time there's too much going on and they are too confusing to work with and you'll be dividing them up into easy chunks anyway.
Basically, any mid-range PC will be perfectly adequate for the majority of DV/rendering work. You can spend loads of money making something specific, but it's not going to give you loads of extra speed.
As for silence... as it's going to be pretty mid-range and not overclocked, maybe look for a pre-build one from a major supplier?
Will guarantee stability and reliability (or at least a man out to fix it), which is more important than 5% more speed when it's a work machine and you've got a deadline to meet.
Make sure the monitor is good!
