I'd hold off on the replacement drive just yet... What bothers me is that attempting to boot from a system disk (CD/DVD) still resulted in problems. The hard drive should be bypassed during this. That makes me suspect a hardware issue, if it so stubbornly refuses to start up. On the other hand, you were able to boot from a Firewire disk...
For now, is it possible for you to try that drive in another (suitable) Mac, to see whether it causes the other Mac to hang too? Or: can you have the Mac looked over at an Apple Store or the like?
As for disk images, you may want to look at
Carbon Copy Cloner.
Congratulations for your wedding day, I hope this issue can be sorted for you both before then!
