Well, for one thing, I want to use my 19" LCD display with whatever I buy. I suppose I should have mentioned that.
That's why I'd take apart the eMac and probably convert it into a monitor/speaker combo and take the guts and throw them into a cheap ATX case. Then I could still use the eMac monitor if I ever needed to (plus, it'd be sort of fun to try).
I'm guessing, however, that the eBay route will likely be the most fruitful, since I could probably find a dual-867 with Superdrive for not much more than an eMac with a SuperDrive. In fact, I had a dual-1GHz PowerMac with SuperDrive just last week, but it had to be returned because of a faulty second processor.