2008/9 Mac Pro wont turn on, front pannel LED is constant white.

Question

My mac pro doesn't want to start up.
I purchased some new memory installed it (and cleaned up some of the dust on the inside of the box) and now the machine won't boot up. I get a constant white LED on the front, no fans, no chimes, no other lights light up (RAM riser boards)?
I have removed the new RAM and replaced it with the old, in an attempt to just get the machine to boot. No luck.
So now the box is back in its OEM state, but still wont boot. I've tried the 15sec unplug / hold the power button down for 5 sec rest and that didn't seem to work.
Any idea what might be going on?
I also swapped out a known working RAM riser from another box and still can't get it to boot up?

Answer 1

Thanks for the help...
I'm pretty sure thats it... MacPro3,1.   I can't really tell as stated above the machine wont turn on so System Profiler is not really an option.
The new RAM is DDR2 and was selected using Crucial's memory selector.  Although, I'm not even using it at this point.  The only RAM in the box right now is the RAM that came with the machine in the original DIMMS. (Riser A DIMM 1, DIMM 2, Riser B DIMM 1, DIMM 2) 1G DDR2 Apple OEM RAM per DIMM. 
The box's basic specs are  2x4Core Intel,  4G RAM, nVidia Gforce.
No, I haven't used AHT...are there instructions / manual somewhere?  Does it use two machines? Maybe I'm missing something here but I'm not sure how to run something on a machine that doesn't turn on?
Edit:
I think its a 2 x quadcore... not 2 x 8. I looked up the system profiler of another mac we purchased with in a few weeks of this one.
Here is the system specs for that machine, which probably match this one.
Model Name: Mac Pro
  Model Identifier: MacPro3,1
  Processor Name: Quad-Core Intel Xeon
  Processor Speed: 3.2 GHz
  Number Of Processors: 2
  Total Number Of Cores: 8
  L2 Cache (per processor): 12 MB
  Memory: 4 GB
  Bus Speed: 1.6 GHz
  Boot ROM Version: MP31.006C.B02
  SMC Version (system): 1.25f4