DDR SDRAM gives about a 30-40% improvement in most apps, according to a look Ace's took at a Micron chipset (the memory controllers are supposed to be nigh identical between the Micron and AMD chipsets). If you consider another 5% improvement from the move to 133MHz/266MT FSB, then a DDR Athlon system should be anywhere from 35% to 45% faster than an SDR solution . . .
Consider it this way . . . a 1GHz Athlon with DDR would be comparable to a 1.35 or 1.45GHz system with PC133!