[Solved] Best course of action for adding 2nd HDD and new OS?

Agree with all above. Note that, to get AHCI running, what you must do is go into your BIOS Setup screens and set it BEFORE installing Win 7. In the right BIOS screen tell it to treat your SATA drive(s) as AHCI devices. AFTER that is set up when you install Win 7 the Install routine will recognize that it is dealing with AHCI devices and install the appropriate drivers. The trick here is to recognize that the BIOS intervenes and "fools" Windows into seeing whatever type of device the BIOS is set to.

I believe that setting the BIOS this way does NOT affect any data on the drive itself, so changing your machine to using AHCI on ALL SATA devices should not make any difference in the ability of the new Win 7 installation to use the old drive. The only difference, as jitpublisher hints, is that if you tried to revert back and boot from and use your old XP installation as your OS, it could not deal with AHCI drives, and to do that you'd need to change your BIOS to use the drives as IDE Emulated devices again.

To keep things clean and simple, I would recommend that, when you start your Win 7 installation, you DISCONNECT your old drive and install the new one on the first SATA port, probably SATA_0, then set you BIOS to use this port as your second-choice boot device (first choice being your optical drive), which may be the way it is set already. Install Win 7 with the new drive as the only drive in your system, setting up Partitions as you wish, etc. When done, THEN shut down and re-install your old drive, but it will probably be connected to a new SATA port. Boot up and verify that it's all there and working, and then you can work on copying data around, establishing where key folders are located and how to get your existing application software "installed" under Win 7.

OH YEAH, quick hint: Windows always sets up its pagefile system on the C: boot drive by default, so I'm sure you have it running on your old drive. When you do the new install it will set its own new system up on the new C: drive. Later when you re-connect the old drive and it gets to be D: or whatever, you may find Windows gives you a hard time about getting rid of the old pagefile files on the second drive. Now, if you're going to be completely wiping off that disk before devoting it to data, that should handle that little issue. But if not, I suggest that JUST BEFORE you shut down and start your hardware / software switch process, you get rid of the pagefile system on your old C: drive. Windows will object because it considers that system important (and it is), but it can run without it. To do this (Windows hides it deep so you can't do it by mistake), click on Start ... Control Panel ... System and then the "Advanced" tab. Under "Performance" click the "Settings" button and choose the "Advanced" tab again. At the bottom under "Virtual Memory" click the "Change" button and just below the middle click the "No paging file" button, then the OK at the bottom. You will have to confirm your silly choice and Windows will tell you you must reboot for this to work. That's true, but you may just be shutting down to start your changes and don't really need to reboot.

Anyone here have an opinion on whether OP's plan to Partition 30 GB of the new drive for the OS in the right size? Seems a bit small to me, but if the plan is to put virtually everything else on another Partition, maybe that's OK.