This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
A new interface may or may not help; it is possible to make a rough prediction if a better interface would really speed things up.
Hardware vendors and marketing people would love to see everyone rush out and buy the latest generation of 'Ultra-ATA' adapters. To achieve this noble goal they tend to juggle with too-good-to-be-true performance figures. The relation between this advertising hype and the real world is shaky at best.
The main point to remember is: a slow drive is a slow drive no matter how good the interface is. If the speed at which the drive physically transfers the data to/from the media is the limiting factor in performance, and it often is, the only way to make things go significantly faster is to purchase a better drive. Note that the transfer modes supported by modern drives (those 33MB/s figures) have little to do with their real-world performance.
In addition, an 'ordinary' ATA-2 (EIDE) interface already offers respectable bandwidth---the fastest ATA-2 transfer mode is theoretically 16.6MB/s, which is more than any Ultra-ATA drive on the market today can sustain. In addition it usually supports the CPU- cycle-saving DMA modes. An IDE interface, on the other hand, makes a much better candidate for replacement since it rarely has a bandwidth over 2.5MB/s, which is cramped by today's standards, and doesn't support DMA.
How to determine if the drive is the bottleneck? You can get a rough idea using Coretest version 3 <ftp://ftp.rahul.net/pub/lps/ disk/core303.exe>. This version of Coretest gives two performance figures of interest here: the (B)uffered transfer rate which is an indication of the bandwidth between drive and interface, and the (S)ustained rate which is related to the speed of the drive media. If your drive has a small buffer cache, you may have to use the /B16 option to get the correct buffered transfer rate.
Usually you'll find that the first figure exceeds the second by a considerable margin (say, a factor two or more). This means that the physical properties of the drive itself are the bottleneck, and improving the interface speed any further won't help much. The only thing that may improve performance somewhat is using block mode (using either a BIOS option or a driver). Only if the drive throughput starts to approach the interface bandwidth will you have a fair chance that a new interface will have a large impact on performance, provided the drive supports faster modes than the one currently used.
This is not an endorsement of Coretest as a disk benchmark; there are more reliable ones around, such as QBench <ftp://ftp.rahul.net/pub/lps/hard-disk/>.
Be sure to read question 7.2 to get a more complete picture.