Just do it with Leopard Assist and forget about target modes et al. It is trivially easy, and the process once started is familiar. Alternatively, the OF method is also literally just a matter of typing a handful of characters. Takes a few seconds to boot to the prompt, type, reboot as directed, and you're off. You are not going to do your machine any damage with it.

A Dual 550 Digital Audio that I have around here runs Leopard without coughing. The upgraded graphics card makes a big difference, but what's in an eMac? A Radeon 7500 or some such? Not too bad. As for the processor, a single (and less capable G4) processor eMac may have more problems than the dual DA, but I'd go ahead and give it a whirl.

Once the initial Spotlight indexing is done, you'll get a good sense of its usability. I would defer judgment until then.