[G2 and Desire Z Q&A, Help & Troubleshooting] rohit275:
I'm no expert myself, just an average user who depends on the work of this awesome community to learn a little bit more about how these things work, so take whatever I say with a grain of salt.

I think all that matters for rooting purposes is removing VISIONary as the wiki states. As for how exactly to do this, not sure since I've never used it myself. Since it's just a temp root anyways, I'm pretty sure if you uninstall it and reboot at any time, it will be unrooted anyways. Titanium Backup will not even work without root, so you might as well remove it too (although not sure if you have to). I'm not sure what Shoot Me is. If you're in doubt, removing it before you root and reinstalling might help, but I doubt it's necessary.

The rooting instructions for the G2 are very specific. Do NOT use the same instructions for the Droid from that youtube video. I'd only recommend following instructions specific for the G2 you find on this site. This right here is definitely the best way I know to do it:

http://xdaforums.com/wiki/index.php?title=HTC_Vision#Rooting_the_G2

I'd had some experience playing around with rooting my G1, but I never needed to deal with adb and was also initially confused. It's basically a shell that you can use to communicate with your phone when its plugged in via usb to your pc, using a command prompt or terminal or something. The instructions for setting it up aren't 100% clear from the wiki, but this guide was amazingly helpful in learning how to set it up in Windows:

http://xdaforums.com/showthread.php?t=865685

Once you have that running, following the recommended rooting method from the wiki (rage/gfree) is seriously a breeze. Just follow it carefully and you won't brick your phone. The devs have really simplified the process for us in a short period of time.

This was also a really helpful FAQ that pretty much answered all my initial questions when it came to rooting, if you read through all of this carefully, you'll be able to figure it out pretty quickly:

http://xdaforums.com/showthread.php?t=835777

Also, if you plan on installing a custom rom like cyanogenmod (highly recommended), you may want to back up your copy of swype cuz you'll lose the swype keyboard when you do it. Here are some good instructions on how to keep that:

http://forum.cyanogenmod.com/topic/10225-how-to-keep-swype/

Overclocking is a completely different game. Once rooted, you can use an app like SetCPU to speed up the clock speed on your phone's processor (stock is 800 MHz). I've seen some people able to achieve speeds of 1.9 GHz or so, but what's more common is around 1.4 GHz if i'm not mistaken. Obviously, for the increased, performance, you'll take a hit in battery life and such, you probably dont want to push your phone's cpu too far if its not necessary. Rooting gives you the option though.

Lastly, if you want to unroot, there are more complicated methods posted if you use the older rooting method which is currently not recommended. For this method, you can basically follow the instructions in this post:

http://xdaforums.com/showpost.php?p=10349347&postcount=81

from this thread:

http://xdaforums.com/showthread.php?t=835971

The PC10IMG.zip can be found in the original post of that thread. The instructions for turning S-on via gfree are also found in the wiki.

I know that's a lot, but hopefully it will answer a lot of your questions.