There arent any real performance issues when using a divider with amd cpus. You probably do gain some performance but unless you can keep up low timings at high frequencies (by using UTT/BH5--basically the high-voltage ram) you'll probably be pretty much at the same performance if you go low-frequency, tight-timings over high-frequency, loose-timings.
About Intel... well, I've heard that having a 1:1 fsb/dram ratio is important but I havent touched a p4 in a while and back then I didnt really thinker with the systems.