OT: Which is broken?! (computer part)

[Instar]

Ok, I've been thinking about my broken computer for almost 3 months, and I'm still not sure which part is actually broken.

To set this up, here are my specs:
MSI 845 Ultra ARU mobo
1.8 Pentium 4 Northwood
Leadtek GF 4 Ti 4400
Creative SB Audigy 1 Platinum
1 gig of DDR
Plextor CDRW
Toshiba (?) DVDROM
Generic floppy
Generic 100/10 ethernet card
80 gig ATA drive (7200 RPM)
2 x 2 gig ATA drives on an on-board Promise RAID controller

This setup ran fine for a long time, no problems. However, at the end of Last semester, it started going wonky at bootup (random stops during POST or whatever). After I brought it home, it lived for about 2 weeks and then finally bought the farm (when I press the case power button, all the system fans spin up, but no beep, and no display on the screen). Mind you this computer is only about 2 years old at the most, so I'm kinda miffed. Now, I've checked the video card (swapped in a PCI card), removed all unneccessary parts (still won't boot), checked the powersupply (on another computer and with voltmeter), the RAM is good (less than 6 months old). I've pretty much narrowed it down to CPU or mobo.

Is there any way to tell which is broken? I don't have a known good mobo or CPU to try, so that's hampered me (this machine I'm using right now is a "slocket" P3).

Arg... any thoughts?