Well, it depends.

For Mac OS 9.1, you can use Photoshop 7.0.1. Photoshop 6.0 will run on Mac OS 8.5 to 9.0, and 5.5 will work on Mac OS 7.6-8.1 (5.0 will run on System 7.5.x). This pretty much covers the range of OSes that your machine will run (your Mach V 9600/350, I presume?)

Here's where I got this: http://en.wikipedia.org/wiki/Adobe_Photoshop_version_history#Version_compatibility

I hope this helps!

c