Set it to AHCI mode before you install Windows. IDE mode is for older operating systems like XP that don't support SATA directly without additional drivers. AHCI mode enables some additional features that IDE doesn't support.
Also - I highly recommend that you disconnect the HDD so that ONLY the SSD is attached when you install windows. If you leave the HDD attached then Windows will place a "System Recovery" partition on it and the system will actually boot from the HDD first before loading the OS from the SSD. That means you won't be able to boot if your HDD dies or if you change it out for a new one some day.
Once Windows is installed you can connect the HDD and then boot Windows to partition and format it for use as a data drive.
That's great info around not connecting the HDD's before installing the OS, I hadn't thought about that and would definitely have hooked them both up this weekend when I go to do the install on my new build.