I'm not running Giga, but I've been running "mini PC" for Kontakt and other VI's for about a year now. It's an Athlon 3200 with 1.5 gigs of ram.
It has one PCI slot which has a wavecenter card for ADAT & MIDI I/O, and 2 hard drives (one is monted where the DVD/CD drive is supposed to go).
One thing you have to really watch is the temp! these little boxes can run hot. I had mine in a little "cubby" closet and it would overheat unless I opened the door. This of course defeated the purpose of the "cubby" for noise reduction. FYI my dual 1 gig G4 runs fine in an identical "cubby".
Maybe that won't be an issue for you with a seperate machine room, but it's something to keep in mind.
FWIW, I've been using my rack-mounted Shuttle barebone for a couple of years now both on live gigs and in a studio environment serving as an extra Giga box. No heating or other problems with that setup whatsoever. The thing to note is that many mATX mobos or barebone models are limited when it comes to RAM as they often only support 2GB.
I have been running six Shuttle SN45G SFF XP 3200 computers for a while now and they have been very reliable.I changed out the exhaust fan for a quieter one,but that is all that was needed.They have the Silent X power supply and I use MIA cards inside those boxes and feed the mixed digital signal into some Roland M1000 mixers..The Giga versions have an 80GB boot drive and a 120GB read drive-all WD JB 8 MB cache drives..The Sonar boxes have been running 2 x 120 GB..The chipset is an Ultra 400 which is the fatest FSB spec of an XP board available...All that considered though-I have just switched over the main computers to nVidia Ultra Sli MBs with Athlon 64 x2 4200 CPUs and the WD 3200KS drives-16 MB cache..vroom vroom..The reason is for those SATA drives and I need more room in the case if I am to have a RAID array running.
I am using the Antec Sonata II cases with as many passive components as possible and there are more choices available in a standard configuration.The SN45Gs are powerful little boxes-but I have a feeling that they will all get turned into some of the Athlon 64 4200 x2 boxes..The new boxes are not as small and cute-but they are very quiet and they crunch numbers extremely quickly..With GS3 Orchestral-one of the new boxes can do what 2 or 3 of the others were doing.. That equals less noise-and that's nice ajw
we are working GS 3.12 on a MSI Hetis 865 GV equipped with an Intel P4 3,4 GHz, 2 GB Ram and an SCSI Harddisk. OS is XP Sp2 in "acpi multiprocessor" mode. The system is stable with 3.12, was unstable with 3.10. The only problem is that there are some clicks when the machine gets "hot" - temperature.