Here's a quick summary of the difference between the cheap RAM and the good stuff: Stream memory bandwidth scores rose considerably, while Winstones added a point or two. Quake III scores jumped nearly 20 points, and 3DMark 2K1 scores went up almost 200 points. In other words, the difference is nothing to sneeze at. If you're buying PC133 SDRAM, buy the good stuffthe 256MB stick I bought was only 46 bucks. (Personally, though, I'd be buying PC2100 DDR memory.)
That said, the PC133 SDRAM-based "Morgan" Duron still smacks down the PIII in Stream tests by about 140-150MB per second. Both chips have hardware prefetch logic to boost memory performance, but the PIII is stuck riding a very slow bus.