pastorfish, not to take anything away from the excellent advice you're getting from ace, but I'll add a couple of points.
Just in case you haven't seen it yet, I highly recommend you take a look at this post by Scotty85 s-on and want to flash roms? READ THIS - Android Forums. It contains some of the details on why being S-On impacts the ROM flashing process. Also in that post, he describes an alternative to having to strip the boot.img file out of the ROM zipfile and flashing it separately via fastboot. I think it makes things simpler.
Also, once you've flashed your alternative ROM and let it finish, it's almost always a good idea to shut your phone down and re-boot first before deciding that things didn't go well. Sometimes it takes a second boot to set all the ducks in a row. Note, though, that if you re-boot it from the off state, you'll go back to the stock recovery and would have to re-boot your alternative if you wanted to do something like a nandroid backup, etc.