It all depends on your traffic pattern. Comparing RPM is not really relevant because the maximum throughput depends on the platter density. A 3-4 years old first gen green 5400RPM drive would top out at 40MB/s at the very end of the disk in most ideal case, even lower if it is a 2.5" drive. In the most ideal case 2x 7200RPM drive will just about enough for 1Gbit provided the data if it is a really big drive say 1.5T / 2T / 3T.
Random access will slow the drive down significantly. For example 10 users downloading together you may see the 2x7200RPM drives spit out 600Mbps only.
300Mbps is normally fine with 2 drives unless you are doing bittorrent.
It depends a lot on the number of simultaneous transfers and other factors.
The operating system caches files read from disk drives into memory automatically.
So for example, if you have just 3-4 GB of content, this content will most likely be eventually cached into RAM and you will saturate 1 gbps line.
You could also saturate 1 gbps line with just 20-30 simultaneous file transfers, let's say some Linux 4-8 GB iso files.
On the other hand, if you have 300-500 users each downloading small files, your hard drives may not be able to keep up with seeking in various parts of the drives to retrieve the data, therefore your total throughput could go even lower than 100 mbps.
So the conclusion is that rpm doesn't mean much in regard to bandwidth - it's more of a factor of disk access and latency (how fast a certain file can be found and transfer started from disk)