Before we get too far here, you do understand that you don't need to flash a ROM of any kind if you don't want to, right? You can root, and keep the stock ROM, and everything will work. Should you choose to flash a custom ROM, and want to keep everything (4g, HDMI, etc.), you can flash one based on Sense. The ones that don't have working 4g (but will soon) are based off what's called AOSP. That means it's based off Google source, and has nothing to do with HTC code. CM6 is just one such example. It just happens to be the most popular. If you've heard of MIUI, that's another example. In any case, get comfortable with rooting, first, then you can deal with flashing ROMs later.
As for the root process, you said you read the Rooting for Dummies sticky. Do you have specific questions about it?