some times? your always abrupt...
you spend more time putting all the best than giving answers...
if you dont have time then save your energy and put what you save into something else...
anyways OP theres not a lot you can do your max is just old and below spec for the game. its over 6 years old so you would be lucky to run any recent games on it. they did cost a lot back in the day but they were very under spec as far as a comparative pc went. diablo 1 isnt actually supported on your hardware. it may run the o.s fine but games are different they have much more need for set requirments and the g4 just doesnt have em...
the gfx core in the g4 doesnt meet the minimum direct x requirments so there is no way it will play...
Classic won't work since you're running an Intel. Diablo under Classic (on PPC) works poorly or not at all anyway, in my experience.
You can try SheepShaver. However: it can be confusing to set up (the linked directions @Xeem are pretty good); you need an OS 8 or 9 system folder/disc to run it (NOT downloadable from Apple); and it's pretty unstable for gaming in any case - this on my PPC, which runs it natively.
The Windows solution. Since the Battle Chest CDs are hybrids, you'll have a PC version to run. However, you'll have to buy a copy of Parallels/VMWare. Why not just do Boot Camp for free? Even then, you'd still need (have to buy?) a copy of Windows XP. A little pricey to run one old game.
Short answer: there's no one solution that's both easy and free, sadly. (*to quote from another site that answered your issue)
if you want to play games than you gonna need to spend 500+ on a desktop or 750+ on a lappy. and that will get you an entry level gaming system with enough grunt to play diablo3.