Can I get back to root-able HBoot version by doing a factory reset I have a HTC Wildfire (Buzz) that originally came with Eclair. I now have Froyo on it and the Unrevoked tool doesn't work as the HBoot version is not supported (too new for Unrevoked).

At this point, I don't care about losing the data on the phone (they are backed up anyway). So, can I do a factory reset to get the old firmware back that can be rooted?

I'm ready to do anything to get Cyongenmod on this :)
No, performing a factory reset won't affect your HBoot version. 

You should, however, be able to follow this guide instead (which uses Revolutionary). There's another page with instructions here which describes in a bit more detail what Revolutionary does and where to go from there. The standard disclaimers with regards to possibly bricking your device apply, of course, and I can't guarantee 100% that it will work.