Will not boot, grey screen

"this sounds like a possible failed logic board or processor module. Since the 2.5 Quad is water cooled, first check the felt pad under the processor heat sinks - is it damp or stained? If so, stop trying to use the machine immediately - the LCS has failed!

Next, make sure you have a known good matched pair of RAM modules (these machine are VERY picky about matched RAM - try to use not only the same size, but the same manufacturer and code when possible) installed in the inner most slot of each bank - follow the diagram on the inside of the cover to ensure the RAM is installed in the proper banks.

Next, if the issue persists when you boot the machine after checking all of this, remove the cover and air dam and check the LED bank on the forward edge of the logic board, near the RAM slots:

http://support.apple.com/kb/HT2652

Red lights here are BAD (except for the ""cover open LED"" My guess is you will have either the ""Overtemp"" or ""Proc"" LED lit.

There are sources for the parts, let us know what you find..."

"remove as much RAM as possible,

unplug all unnecessary devices (USB/FW etc)

find and use CUDA button http://support.apple.com/kb/HT1939

If this not helps remove battery for some time (ie 1h) then place it again

If none of this help, I'm afraid that you have a hardware problem :("

In same old macs they didn't boot if the clock batery was dead, I don't know if it is the case but check the battery voltage just in case.

"From what you described my first guess would be the logic board. When you are in open firmware and the keyboard does not respond that could be a sign that the ports have stopped responding due to the failure.

My second guess would be the processors.

What I would try to do is try and start the computer using only 1 RAM slot. If you can try and boot to your system disc or your AHT (Apple Hardware Test) CD do that and see what happens.

If none of the above work, I think you'll have to take it to an Apple store to get it looked at.

good luck!"