Since it started up once, it sounds more like a fried power supply in this particular case, but there is a section over at MacInTouch that popped up right after that big blackout hulabaloo on the East Coast a while back that has suggestions for resurrecting computers with symptoms similar to yours.
I mention this because at one point a friend's Quicksilver died on him in a similar fashion, and I managed to resurrect it without replacing anything. What I tried:
Unplug it. Pull the motherboard battery (you've already put a new one in, which is a good thought). Push the power button on the front to drain any residual charge (it usually lights briefly and the fans'll twich). Push the motherboard reset button just for good measure, and you can even let it sit in this state for a while, to be sure everything is really dead. Then, re-plug, re-insert battery, and push the motherboard button briefly, once only. See if it starts up while holding down command-option-o-f, and if so do a reset-nvram and reset-all. Some Macintouch folks recommended leaving it unplugged for a full 24 hours, but that wasn't necessary for the one I fixed.
In your case you've done pretty much everything in this dance, and it still didn't work, so it's probably the PSU (at least, if you're lucky that's all that's wrong).