How does it tell you it needs a new OS?

I'm thinking System 7.5.5 through Mac OS 8.1 would be fine on a 610.

7.5.5 is free and stable, and probably the best bang for the buck. 7.6(.1) is very stable and can do much of what OS 8 can do without some of the extra bloat (like new Finder features such tabbed folders, etc). OS 8 has more features and is generally more modern, but it isn't free and requires at least 12 MB of RAM.

Peace,

Drew