i845 drivers are not needed for Windows XP, although Intel does provide them if you need them.
The inf drivers simply describe recognition to the OS, and make no difference on performance whatsoever (so long as you have them installed). If they are not present on an OS that does not natively recognise the chipset, there can be a fair difference in performance.
The Intel Application Accelerator drivers are a re-jigged version of the ATA Storage Drivers, although they are much more stable now in it's latest version, 2.0. They make a 10% difference to Win2K, but with the aggressive pre-caching and parallelism built into the Microsoft drivers, the IAA are not needed (for the moment). I haven't had a chance to benchmark the difference, but I could not see any difference under informal testing (on a Pentium III). The IAA are supposed to provide bigger speed gains for the P4 over the P3, but as I said, no one has benchmarked it.