After much thought, a lot of numbers-crunching, and a long, thorough talk with my mechanic, another mechanic in his shop, and a couple of guys at work who restore cars on the side, I've decided that it's worth repairing Sapphy. All her other systems are sound, I have more than enough in my home equity line to handle the repair, and she only has 108,000 miles on her.
One more biggie and I'll have to replace her, but at this point in my life, it's worth fixing her. So that's what I'm going to tell my mechanic tomorrow. However...I'm going to keep an eye out for good deals and good financing, and start researching alternatives. Better to be safe than sorry!