If you're looking for consistent speed for accessing data from external hard drives on your Mac avoid USB 2.0 if at all possible - use Firewire or eSATA (if you have a MacPro or Power Mac G5) instead.
While USB 2.0 has a higher theoretical throughput than say Firewire 400 (480 vs. 400) it's real world throughput is less and is "bursty" which can result in dropped frames quite easily. Firewire and eSATA won't have that issue.
Anyway a quick rundown for theoretical speeds you can get from the different connections:
USB 2.0: 480 Mbps (~60 MB/sec)
Firewire 400: 400 Mbps (~50 MB/sec)
Firewire 800: 800 Mbps (~100 MB/sec)
eSATA 150: 1500 Mbps (~188 MB/sec)
eSATA 300: 3000 Mbps (~375 MB/sec)
Fibre Channel: 1000 - 10000 Mbps depending on installation. (Expensive)
Ethernet: 3 - 100000 Mbps depending on installation and overhead. (Cheap to exceedingly expensive)
Future / Extremely new Protocols
Firewire 3200: 3200 Mbps (~400 MB/sec)
Firewire 6400: 6400 Mbps (~800 MB/sec)
USB 3.0: 5000 Mbps (~625 MB/sec) (with protocol overhead)
3200 Mbps (~400 MB/sec) (actual throughput of data excluding protocol overhead)
eSATA 600: 6000 Mbps (~750 MB/sec)
Those are the theoretical speeds that a single connection can give you - whether or not the hard drive(s) you have connected can saturate that link is a different story. For example a 5400 RPM drive
The theoretical maximum data transfer rate on a hard drive can be determined by dividing the sectors per track * 0.5kb divided by the revolution time. (Source: IBM) Which is why you'll find larger sized hard drives will actually give you better transfer rates (more data per sector). A 7200 RPM drive should be sufficient for most applications depending on what file you're attempting to read and how fast. (eg. It won't be sufficient for Uncompressed HD, REDCODE, etc.)
The other way as specified before is to set up a RAID in order to attempt to saturate the link you have between the hard drive(s) and your computer. RAIDs, fibre channel networks, or even 10-gigabit (or even 100-gigabit) ethernet networks can get very expensive very fast but can offer speeds not possible with more normal connections.