Okay so we know it isn't the monitor. Could either be the motherboard or hard drive. Try putting in an XP CD to see if it boots to the CD (or Linux live CD).
If not (and since it isn't showing the POST screen) i would presume the video card probably went out, which is integrated into the MB.
So now it would be a safe bet to replace the MB. Same conclusion you came up with yesterday, but now you can be almost certain this will fix the issue. Would have hated for you to spend 200+ on a motherboard, 4 hours replacing it, just to find out you need to spend another 200+ on a monitor to replace that.