The Intel Application Accelerator is only for Intel 810 chipsets and higher, and is available for Windows 98 through XP. It will basically upgrade the drivers for your IDE interface and offers performance enhancements to increase hard disk access. It is not required, but is recommended. Here's what the Intel website < support.intel.com > says about it:

The Intel® Application Accelerator is a performance software package for desktop PCs using select Intel® chipsets. The following sections describe the major benefits of installing Intel Application Accelerator.
Enhanced System Performance
Intel Application Accelerator reduces the storage sub-system bottleneck, enabling faster delivery of data from the hard drive to the processor and other system level hardware.
Improved Application and Game Performance
Intel Application Accelerator provides accelerated disk I/O for games, graphics applications, disk utilities, and media authoring applications.
Optimizations for Intel® Pentium® 4 Processors
Intel Application Accelerator enables a performance-enhancing data pre-fetcher for Intel® Pentium® 4 processor-based systems.
Faster Boot Time
Intel Application Accelerator delivers faster overall system boot times by significantly accelerating the load time of the OS - enabling you to build Pentium 4 processor-based systems with a better overall end-user experience.
Large Hard Drive Support
Intel Application Accelerator supports 48-bit Logical Block Addressing (48-bit LBA) for 137 GB and larger hard drives. For additional information, go to the Large Hard Drive (48-bit LBA) Support page.
Automatic Selection of Highest DMA Transfer Mode
Intel Application Accelerator automatically enables the highest transfer mode supported by the ATA/ATAPI device and Intel chipset. For example, a CD-RW drive that supports a maximum of UDMA-2 will be set to this mode with no interaction required. Two devices that are on the same ATA channel can even be using different transfer modes.
Diagnostic Utility
Included in the installation package is the Intel Application Accelerator diagnostic utility that provides detailed information on all of the system ATA and ATAPI storage devices including hard drives, CD-ROM drives, DVD-ROM drives, CD-RW drives, and other ATAPI removable media drives. This utility also allows modification of settings for transfer modes.
Upgrade Path from Intel® Ultra ATA Storage Driver
Customers who are currently using the Intel® Ultra ATA Storage Driver can upgrade to the Intel Application Accelerator for the latest enhancements and updates. Intel Application Accelerator has replaced the Intel Ultra ATA Storage Driver.