OK let's start with the easy questions first.
Easytune6 is not something I or any one seriously into overclocking would use. If you wish to use software for overclocking then AMD Overdrive is probably better. When Overclocking it is much better to do so from within BIOS. Although it takes longer to reach your targets you have much more control and therefore can choose what is best for your system.
Clearing CMOS is fairly easy and you can find how to do that here:
http://forum.giga-byte.co.uk/index.php/topic,2286.0.html The question of CPU Speed vs the amount of Voltage to achieve it, is something only you can eventually decide. The faster you go with the processor usually the more Voltage is needed. The problem with this is that the more Voltage you use, the hotter things become and the shorter the lifespan of the components you're stressing. (Back to one of your original questions there.) Getting 3.8GHz at 1.35V or thereabouts is a nice target as it gives you a speed increase but you are still well within the Voltage range of the CPU.
Getting 4.0GHz or above you may need to increase the Voltage but this should be done gradually, in small steps. Yes, your system will fall over (BSOD) until you find the optimum settings but whacking things up to a predetermined amount could lead to you further issues and is not advisable.
The best advice I can offer is that you leave your voltages at default (AUTO) while you increase the CPU Multiplier. Doing this you should be able to reach 3.8GHz without too many problems and your voltages should remain fairly low. Once you have established the 3.8GHz make a note of the voltage that it is running at and then manually set this in BIOS so it doesn't alter. If everything is stale you will then have a couple of choices.
Gradually reduce the voltage on the CPU run some tests and repeat the Voltage reduction test cycle until it falls over. then return the voltage to the lowest fully working setting.
Start to increase the CPU Multiplier and test. If the system BSOD's leave the multiplier alone and slightly increase the Voltage. Test again. This process can be quite time consuming and is not something to be rushed. What you're trying to get with this is the fastest CPU Speed with the minimum working and stable voltage. Remember though, as you increase the speed of the CPU Multiplier you will also probably need to tweak other settings in BIOS such as Northbridge frequency and voltages.
You will also probably have to keep an eye on your RAM Speed as, when you increase speed in other areas this too will increase. If you haven't done so then I would advise you to read this:
http://forum.giga-byte.co.uk/index.php/topic,2515.0.html with reference to RAM and your CPU.
