Just to add some detail to this thread:
We have not switched to Vista, so the following applies to WinXP and only applies to SATA drives as far as I can tell.
Dell has started shipping their desktops with SATA AHCI enabled. Previously, the default was SATA ATA. The difference seems to the be that AHCI allows more efficiency for the drive as well as hot swap ability. Interestingly, what I was able to dig up suggests that in a lot of cases AHCI will actually degrade performance, but my impression was that the margin was very small.
Why does this make a difference? Well, where I work we use Ghost to install preconfigured images to new computers. With AHCI I was unable to boot from the Bart PE CD from which I run Ghost. In fact, I couldn'ty even boot from the Dell provided Windows XP installation CD. Turns out, CD drives (even SATA) don't understand AHCI - or at least not yet. If you try, you'll get a BSOD!
The work around was to change the BIOS setting for drives to the ATA mode, just before creating the image, then resetting after image creation so that the machine can boot as normal. When it's time to install a new image, the same thing - change to ATA mode before installing and then back to AHCI just before booting the newly setup computer.
What happens if you don't reset to AHCI before rebooting? BSOD! This will not occur on Windows Vista systems. Vista *includes* AHCI drivers by default, whereas WinXP does *not*.
Final word - I was unable to observe any functional differences between the computers operation when in AHCI vs. ATA mode.