You don't need the VIA 4 in 1 drivers, since the K7N420 uses a nVidia chipset. The drivers you need come on the CD that is in the motherboard box. Just load Windows, drop the CD in there and you will see a list of items. I believe the first one says "Load Chipset Drivers" or "nForce chipset drivers", just load them and you'll have to reboot a couple times. Next, do the second thing on the list (if you plan on using the onboard video), which is "Load nVidia graphics driver" or something like that. (You can also use the new Detonator XP series on nVidia's website.)
After that you should be all set.

If you want you can load the PCcillin antivirus it comes with (on that same CD), and the PC monitoring software.
There aren't really any performance tweaks that you need to do, the board seems to perform well right out of the box.