Unknown_K is right. The 250MHz, 300MHz, and 350MHz 604e CPU cards require a different motherboard power voltage, and they have built-in "inline" L2 caches. Both of these features make them incompatible with the 8500 (and all other similar Macs, except the special 8600s and 9600s with the "Kansas" motherboards).

There are a few 233MHz 604e CPU cards out there (because Apple did produce a 9600/233) - but for anything faster than that, you'll need a G3 card in your 8500.

The good news is that even the slowest, and therefore cheapest, G3 card will make your 8500 faster than even a 233MHz 604e would.

Best,

Matt