Considering that it was used, the factory restore was a good idea. That way, you can be absolutely sure the previous owner's files and modifications are cleared out. Assuming the hardware was all in good condition, this gave you a computer in its brand new, out-of-the-box condition.
This is not the same as a clean install, as it includes all the pre-installed Asus software. If you want a "clean" machine, you need to re-install Windows and all the essential drivers, replacing the installation you have now.
If you just want to tidy up the installation you have, there are two main tasks: (1) uninstall as much of the Asus software as you're willing to let go, and (2) update some of the drivers.
Once you decide how you want to proceed, we can offer some guidance. (Regarding one question you asked....you don't just download and install everything you see in that Drivers thread. It includes a lot of optional software, and drivers for other models.)
EDIT: The G74 doesn't have a BIOS that allows overclocking. But you should focus on optimizing your setup to your liking before considering things like gaming tweaks.