i will suggest that you create a set of recovery dvds/isos for your stock windows via ai recovery first. this is the only chance you can save a few bucks, after which you will have to purchase it from ASUS estore.
to have a fresh install, i will explain a bit more, based on what madnj suggested in the post above:
1. grab the latest drivers from ASUS Website
- if you don't have the windows 7 installation disk, grab the relevant ISO also.
- save all these files into an external storage. create a windows 7 install dvd/usb.
2. set up your bios
- you will need to make sure that AHCI is selected in your Bios > Advanced Boot > SATA Configurations; you might also need to disable UEFI Boot if there is such an option.
3. prepare your SSD
- you put the new SSD in the secondary bay, and boot your machine; so that you can upgrade its firmware. to do this, you need to get the latest SSD management software from your SSD manufacturer website.
4. fresh windows installation
- first, replace your stock HDD with the new SSD (i.e. put it in the bay where your stock HDD is placed), and then load the windows installation.
- don't put in the stock HDD in the secondary bay yet (take it out of the bay physically). normally we will keep the stock HDD for a while, until the new system is stable.
- if your windows failed to boot, check and make your SSD the first boot option in bios. make sure you take out your installation media upon first reboot. normally it will boot straight away, because it's the only bootable in your system.
5. driver installation
- install the drivers. multiple reboots are required in this stage.
6. run Windows Performance Index
- from what i've read, this triggers SSD optimized settings in windows 7 (e.g. enable TRIM, disable prefetch; superfetch, etc.).