I recently did the same thing, it was relatively easy. I hit a few snags along the way, mainly with Windows 7, just make sure you download and burn the service pack 1 version ISO onto a CD. After you install your SSD (search for an online video and watch it first), make sure the BIOS sees and then you can just plop in the Windows 7 ISO CD boot to the CD and then go through the setup process to install it to your new drive, pretty easy.
It is also highly recommended that you get the important drivers (Chipset, Wireless Lan, Lan, Graphics, touchpad etc... and put them on a USB Flash drive for immediate install. I was also having some issues with the bluetooth and USB drives which was solved by inserting the CD that comes with the laptop and picking only the ones you want.
Here is my thread from last month:
http://www.asusrog.com/forums/showthread.php?5361-Installing-SSD-with-Windows-HP-64-on-an-ASUS-G74SX...
G74SX
-Intel X25-M 80 GB Mainstream SATA II SSD
-Intel Centrino Ultimate-N 6300