Because there are so many tedious little details about getting the whole rooting job done, it is easy to get into a situation where you "can't see the forest for all the trees".
It helps to think of the rooting process as only three fundamental operations:
[SIZE=+2]
1) Get the 1.49.2000 S-OFF bootloader on to the phone.
2) Get Amon_RA's custom recovery on to the phone
3) Put the ROM of your choice onto your phone using the Recovery menu [/SIZE]
Step #1 is the "easy" part - in principle, you don't even need a computer for it, because you could just download the ROOT PB00IMG.ZIP file right to your phone using the phone's browser (and maybe a file manager to move it out of /sdcard/download to the root folder /sdcard).
Step #2 is where everybody seems to run into trouble - mostly because they have a driver install problem, or are not familiar with the Windows (or Unix) command line. But it is important to keep in mind: the goal of that step is to get Amon_RA's recovery boot put onto the phone; that's really the only goal of this step.
Step #3 is also easy; it determines which ROM you put on your phone. You can put any (Eris) dev ROM on the phone at this step - you don't need to start with a specific ROM to begin with.
It might help to re-read the guide, keeping in mind the "big picture" of those three steps; I think if you do that, the whole process will seem much clearer to you.
I hope so, anyway.
eu1
PS Search this forum for the phrase "fastboot flash recovery", and you will see an alternate method to all those adb and Unix command lines - once you have the HTC drivers installed on your (Windows) PC and working correctly, it only takes a single command to flash Amon_RA's recovery (step #2) to the phone.