Assuming this is a custom build and not any premanufactured system you would need to look at the post screen with the logo screen option disabled in the advanced section in the bios setup or user manual for the board in order to see which F key is used.
On a custom case Asus boards would typically the F8 key used at post while it would be the F12 for a Gigabyte board. The bios used whether Award, Phoenix, AMI will determine that.
Here's the first few steps to follow in order to get things right.
1) Clone direct with Acronis to the new host drive or use the system image option in Windows for the original drive.
http://www.sevenforums.com/tutorials/663-backup-complete-computer-create-image-backup.html
Note the intended second drive will need to be used to store a full image there for restoration afterwards since you can't restore an image to the drive you are storing it on.
2) Replace the original as first drive with new drive in first sata port and set as default boot device. Note you can often simply switch the drive end of each data cable to reassign ports without much fuss to simply things.
If you created a full image you then slap a quick install of 7 on the new host without worry of activation or going through the full install but long enough to restore the image made.
http://www.sevenforums.com/tutorials/675-system-image-recovery.html
You first want to see the new drive in working order before the next step. Whether a direct clone or restored image option was used the new drive should see 7 running strong without problems. The second drive will be the temp storage drive until the new main is up and running well.
3)Once the full image is made or first drive is cloned and the new host is the present host drive the original drive is replugged in as the 3rd drive. If you elect to use the fast install to restore an image through Windows following the guide the new host will need to be Drive #1 for the Windows installer to place the boot files and mbr info there.
Booting live from the 7 dvd is the other option that will see the new host listed as Disk 0 as well where it will also detect the latest image. If you boot live from the 7 dvd to restore an image leave the new 3rd unplugged since the repair tools will detect that installation if you haven't added a fast 7 install on the new host. This will insure the image is restored to the intended drive even without a temp install of 7.
4) With the new host/boot drive running well you can now use the original drive to store an image made to protect the new host in case of any need to replace the cloned or reimaging of 7 there. The builtin tool has been found the most reliable for restoring host drives!
Since you couldn't store an image on the first drive from the start or opted for a direct clone you now have another use for that drive. You may even elect to reformat it in order to store the host image and if planning to try restoring a working image to the second the image made with Acronis.
5)Once the first four steps are met you create a full disk image using a 3rd party program for restoration on the to be second drive and store that on the original. From there you use the recovery options in Acronis verifying which is the "destination drive" by looking at the drive indicator which lists the drives installed.
One tip here is labeling each drive ahead of time as the 7 host/boot, test drive, and new storage drive in order to avoid confusion when using clicking on the listing for the destination drive(then 2nd HD) before clicking on the "proceed" button. You will also see the "0 sector", "100mb(if originally seen)", and the main 7 primary listed on the next screen.
Acronis will list things a bit differently then the builtin system image and recovery options listing the "0" sector item or mbr while the 7 tool shows checked off items for the 100mb and C primary.
6) With the main drive running normally you may want a fresh single primary seen on the second drive rather then one left "raw". A quick partition and format will allow Acronis to see the destination drive when going to restore that image onto the new 2nd drive. You will still see the warning about any previous files and data being overwritten and can ignore it since you won't have anything there to worry about yet.
7)Lastly is the actual test of the second drive to see if the image went on well where you can test each drive in progression using the boot device menu anyways. This test will insure the second drive can boot on it's own and show the restored image was successful when seeing 7 running there.
Once each drive is found working it's now time to decide on the simple addition of the boot entry for E, F, G depending on which free drive letter you set for the second drive in the host drive's DM tool you add the new entry for that as a boot option. Note you can still select either drive from the boot device menu at any time with or without the boot option seen when starting up.
The one thing to know right from the start here however is that setting up a dual boot with two separately created drive images was experimental and not a "work always" proven method. What was found was that restoring a single image made by only one program or with the backup option in 7 could not be configured as a dual boot.
The use of the backup feature already included in 7 is competent to backup the host drive while it took an image created by Acronis to see results when restored to a second drive. Paragon, Reflect, Ghost, and other programs might also see results while not having been used for testing this type of dual booting.
In any case the first thing to remember is to maintain the new host drive since that is your main system drive as far as OS is concerned while the rest is "at your own risk"! Be sure to have a suitable place to back things upto beforehand in order to have a "disaster recovery plan" inplace.