Welcome!
1- If you root your phone and keep the same ROM, then you can factory reset to the original settings for that particular ROM.
2- If you root and flash another ROM, then it overwrites your existing ROM and factory resetting will default the phone to the defaults for that specific ROM.
3- To "Unroot" your phone once rooted can be done by flashing an original HTC Rom.
4- If you root while still on 2.1 and update to 2.2, you will loose the "root" and the ability to root again, unless you "downgrade" your Hboot to 0.82 again, but there are programs out there that claim to be able to root the Desire with the latest HBoot, but I can't say because I haven't used them.
5- The Froyo update can be done without Rooting the phone, there is a link in the first few pages of this thread.
With regards to the signal issue, that is very much related to radio version etc., which you can only upgrade or change once you have rooted, but doing so and it turns out to be an actual problem with the phone hardware itself, could mean that you invalidate your warranty.
I would suggest that you hop over to
http://forum.xda-developers.com/forumdisplay.php?f=594 and have a read through all the tutorials there, will help you decide which direction to take with your phone.