Not having a monitor connected and not having a video card at all are two different things. MaxMallan, are you saying that if it won't work sans video card, you don't want to build the system at all? Now I'm really curious about this "very specific application"
It's essentially an audiophile music player. There are companies that sell exactly what I need, but I don't want to shell out thousands for an off-the-shelf product when I think I can make one for 1/3 the price.
The two essential constituents are reducing electrical noise and software overhead. Software overhead seems to be the simple part (there are guides on how to strip pretty much every service from XP). Electrical noise is what I'm focused on. So I need an SSD (instead of a HD); a very high-end, clean fanless PSU; a fanless heatsink (after all optimizations are made); the CDROM drive unplugged from the PSU when not ripping a new CD (machine powered down, of course); no video card or onboard chip (control via remote access), etc.
Would I build the thing if I have to be content with a $2 video card plugged in an unused... maybe. But if I can't get it exact, I might just say and stick with what I got.