With a modern SAS controller, you'll have no problem maxing out 8 hard drives. The fastest 15K HDs can do up to about 200MB/s. A 6Gbps SAS controller (or RAID card) can handle about 600MB/s per channel. Even if you're using a backplane with a single wide (4-channel) port, you'll still have plenty of bandwidth left over.
Depending on what you're using the server for, you may want to consider using SSDs instead of 15K HDs. You could get a good 160GB SSD for the same price or cheaper than the 300GB 15K drive, but much, much better random IOPS performance. The Intel DC S3500 is designed for servers, and is very reasonably priced.