Is a PII300 more powerful than an Athlon 650? If a game needs a minimum PII300 (PIII800 recommended) should it run on the Athlong 650?

Nope, a PII300 is slower than an Athalon 650. The game may be a bit sluggish since an 800 is recommended and you may have to turn off some of the "fancy" features but it should play fine (that doesn't mean it "will" play fine - but it should)

thanks Syd

A 650 MHz AMD is comparable in speed to an 650 MHz Pentium.
It may be a little slower, but it's close.