Thanks for all the tips, everyone. I'm not too experienced with hardware troubleshooting, but I'll do what I can and ask about what I can't figure out. If this information is useful, I haven't made any changes to the system's hardware configuration since I got it (in May), so I can't think of any recent changes that would have made this happen. My best guess is that something failed. Would anyone venture to guess whether it's more likely that the motherboard or power supply went bad? The failure was gradual, but it all went down in about a 15 minute span. First there was locking up right after the OS loaded, then locking up in the Happy Mac screen, then nothing. I'll have to try the more involved suggestions tomorrow; I'll post on how it goes.

Thanks again everyone. This Mac means a great deal to me and I appreciate the responses.