Under Quake III Arena at 640 x 480 x 16, where the performance is purely T&L, driver and most importantly, CPU limited, the Duron performs about 6% slower than an equivalently clocked Athlon.
The performance is still quite respectable, especially considering that the Duron wasn't intended to be a competitor to AMD's own Athlon, rather it was intended to go head to head with Intel's Celeron.
The Duron at 700MHz is just slightly faster than an overclocked Celeron running at 850/100, meaning that it would probably take a 933MHz or faster Celeron running at a 66MHz FSB to outperform the 700MHz Duron, or a 566MHz part overclocked to 952MHz (112MHz x 8.5).
At this high of a resolution the system is entirely memory bandwidth limited courtesy of our test GeForce 2 GTS card, which is why the performance is pretty much the same across the board. It is worth noting that in the case of the Celeron, it is FSB limited before it is completely memory bandwidth limited under Quake III.