Hi Bill - welcome back to TS!
What you are seeing is a kernel panic - which can be caused by hardware issues. Since you've switched out the logic board, that would be my first suspect. Did you replace the logic board with EXACTLY the same one? Where did you buy it?
Did you switch out any RAM? Have you made sure that the RAM is seated properly? (I'm assuming you had to remove it and reinstall it when replacing the logic board - it could be something as simple as badly seated RAM)
Or...there may be something wrong with the hard drive. Try repairing it using Disk Utility (on your install disk - go to Utilities) and let us know what it says. Don't bother with "Verify" BTW - just hit Repair. If there's nothing to repair it will just do the verification anyway.
If the HD checks out ok, you could try reinstalling the OS.
Unfortunately, since you can't seem to get it to start up, it's impossible to see the logs to see if you can isolate the cause. Have you tried starting up in Safe Mode? (Worth a try, though if it's a hardware issue, that may not help)
http://support.apple.com/kb/HT1455Kernel panic info:
http://www.thexlab.com/faqs/kernelpanics.html - there are a number of other hardware related possibilities, including a loose/improperly installed Airport card if you have one...