I don't quite understand the question. It usually depends on the ram. If you mean that to get 200+ fsb is easier async than sync on PC2100, you're probably right. If your using XMS3500 of Corsair or PC3200 from Twinmos, it be too hard, though you probably won't get 250fsb.
However, running asycn isn't recommended. From what I've been told, it slows down the system more than the boost in fsb helps the performance. Sometimes it's unstable to run async also, so you might as well just run the system in sync till you get better ram.