Short answer would be, yes, unless you're running software which takes advantage of the second CPU (which probably excludes 99+% of classic MacOS software) the G3's probably going to grease the 604e at any clock speed in remotely the same ballpark. There are some fringe areas where the 604e can outperform the 750 per-clock but you'd basically have to write your own benchmark intentionally designed to smack the G3 square in the kneecaps to really see a win for the older CPU.

If you *are* running an SMP-aware OS (Linux would probably be your best bet?) and applications I'd still wager the best you're likely to see out of the dual 604e is roughly comparable aggregate performance to, I dunno, *maybe* a 300mhz G3 card? Pretty much the only thing the dual 604e has going for it is the coolness factor.