As I understand it AHCI is required to update firmware etc.
Nope, that isn't completely correct. If you wish to update the firmware on your Intel X25-M Solid State Drive, your best bet would be to set the SATA controller to Legacy / Compatible mode. Though, Intel do claim that updating the firmware will work in AHCI mode on many controllers. However, I would still set it to Legacy / Compatible mode just to be on the safe side.
If you wish to use
HDD Erase (4.0) to restore the Intel X25-M Solid State Drive back to it's peak performance though, you will need to set the SATA controller to Legacy / Compatible mode.
Note: Since the website to get HDD Erase (4.0) which I have linked to above is currently down, you can download the 3.3 version of HDD Erase directly from this link
here.
Once Win 7 is installed on the SSD a fresh install won't permit AHCI to be enabled, it can only be enabled on a wiped drive. Or have I got that completely wrong?
If you installed Windows 7 when the SATA controller was set to IDE mode in the BIOS, if you then change the controller to AHCI mode, you won't be able to boot straight back into Windows 7 without running into problems. Just to add, as Jimbo Mahoney mentioned below, you can indeed change the controller from IDE to AHCI mode without having to re-install Windows. If you wish to do this, take a look at this page
here.
If you already have Windows 7 installed and wish to do a clean installation for various reasons, if you boot from your Windows 7 media, continue to go through the Windows 7 set-up until you can delete the relevant partitions. Once you have deleted the correct partitions, restart your system and go into the BIOS and set the SATA controller to Legacy / Compatible mode and then use HDD Erase to restore the drive to it's peak performance. After the process has completed, which doesn't take long, restart your machine and then go back into the BIOS and set the controller to AHCI mode. After you have completed those steps, then simply re-install Windows 7.
If you have the Intel X25-M G2 Solid State Drive, Intel should be releasing a firmware update, presumably shortly after Windows 7 has been released, that includes TRIM support. Once Intel have released their new firmware and you have downloaded it, go into the BIOS and set the SATA controller to Legacy / Compatible mode and then flash the new firmware. After you have flashed the new firmware, go back into the BIOS and set the controller back to the mode that it was on before you flashed the drive. The firmwares that have been released for the Intel X25-M G1 and G2 Solid State Drives previously haven't required the user to wipe the drive after flashing it so the user can simply boot straight back into the operating system that they have installed, as opposed to having to do a clean installation of Windows. This should be the same for any future firmware updates as well.