7.5.5 is buggy and crash-prone; OS 8.1 should run pretty much any 68K game bar the older B&W and early colour games on an 840AV. It also supports large HFS+ volumes, and your 840AV should be kitted out with a nice fast HD
7.6 is also a consideration, but no HFS+ support.
7.6 is also a consideration, but no HFS+ support.