Yeah, currently the fastest drives usually peak at 40-50 MB/sec. So you won't notice much difference using ATA 100 over ATA 66. However, it is important to note that having the extra overhead can actually improve overall performance. It may not be "faster" at any given moment, but the overhead helps the system be more responsive over a period of time. In other words, there is less of a bottleneck.
Also, you shouldn't be too concerned about having to buy a hard drive that supports ATA 100 right now (although almost all newer drives do). The main concern should be having a motherboard that does, for drives that will eventually become faster than what the ATA66 standard allows.
tek