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It sometimes varies with computers. If you can use AHCI, this provides the best performance but may have compatibility issues. RAID is usually only used if you have multiple drives. IDE is the most stable. My answer would be: Whichever gives you the best performance and least compatibility issues.
I was unable to find anything in sevenforums related to the RAID AHCI mode, but I did find this on MicrosoftAnswers.
"Changing from RAID to AHCI, you need one more step. Msahci driver requires Atapi.sys miniport to be loaded during boot as well. So you need to change theStart value to 0 for both of the following entries in your registry. You can use regedit.exe utility.
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\msahci
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\atapi
Or you can also simply run the below command lines in an elevated window.
REG ADD HKLM\System\CurrentControlSet\Services\msahci /v Start /d 0 /f /t REG_DWORD
REG ADD HKLM\System\CurrentControlSet\Services\atapi /v Start /d 0 /f /t REG_DWORD"
If you change from RAID to AHCI and your computer will no longer boot, you may have to do a repair install of Windows 7. See Repair Install
Edit: Make sure you have the right Windows 7 DVD for your computer. You need one with service pack 1 integrated if you have service pack 1 installed. If you do not have a DVD with Service Pack 1 available, search google for official windows 7 dvd iso and you can legally download the appropriate iso file.
Before you go changing your hard drive settings, however, may I recommend one more thing to try (if none of my previous suggestions helped). Look at Optimize Windows 7