How can I enable AHCI mode after Windows Vista install in IDE mode and before/for Windows 7 clean install?

Upgrade to Windows 7 first (as it supports newer hardware).

Then enable the AHCI driver in the registry before you change the SATA mode of the boot drive. To do this, follow these steps:

1. Exit all Windows-based programs.
2. Click Start, type regedit in the Start Search box, and then press ENTER.
3. If you receive the User Account Control dialog box, click Continue.

4. Locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci

5. In the right pane, right-click Start in the Name column, and then click Modify.

6. In the Value data box, type 0, and then click OK.
7. On the File menu, click Exit to close Registry Editor.

After this you’ll have to restart your computer, go to BIOS, and enable AHCI. When you log in to Windows again, you’ll notice the installation of drivers for AHCI. Another restart will be required to finish the driver installation.

Source: MSKB

You should also check the motherboard manufacturer's website for the latest controller driver.