Sorry to say, but it's exactly what happened with my Dual G5 (the Intels - MacPro and MBP - eventually did fine). The installation stalled before it was completely done, the computer froze, and after a force-restart (and getting my DVD out of there), the Leopard screen came on, and on, and on, but it never went past the login/password stage. I ended up having to reinstall Tiger on the 2nd internal drive (which was empty), restart from there, wipe out the first internal drive with the incomplete Leopard, and lose most of that drive contents in the process (which wasn't such a big deal, because I have everything backed up on multiple drives).
So, Intel or not, I only can say: back up!