If you can try an 'older' cpu in your board, it would help...may have some issues with the board detecting the cpu...Or the ram may be causing issues.
Try what you can in the other system to see if they boot the system up.
Do you have other ram to try in your new system? And a different cpu? Would be worth giving them a go if you can.
At the moment your system isn't posting, so it's either your motherboard not detecting your cpu, or your board not detecting the ram properly so it can't display any video. Or your graphics card isn't detecting perhaps....Any older G-cards laying about?
The problem is you don't have a case speaker, so you can't determin if the system is even trying to post or not....
Your problem sounds similar one I had. You have the same RAM as me except I have 2x1Gig. Apparently this RAM is sensitive to voltage settings, my mobo defaulted it to 1.8v when it needs to be set at 2.0v. Until I changed the voltage settings, the computer failed to start up if it had been off for a while, once it had (literally) warmed up it booted up fine.
Your mobo is different to mine, but I suspect that it too is defaulting the RAM to 1.8v or some other voltage that it doesn't like. Either that or the RAM itself is bust. I would suggest swapping out the RAM totally if you can and haven't already. Otherwise you could try what I had to do and turn on the PC and let it sit there for a minute or so on its blank screen to let it warm up. Restart it, if it posts then go into the BIOS and change the voltage for the RAM. For me it had to be 2.0v, but to be sure I would check this thread for the correct settings for your exact RAM and motherboard: http://www.ocztechnologyforum.com/forum/showthread.php?t=27196 (NB the info for your RAM is about halfway down)
Don't take the cooler off no...the most I would do is "rest" the cooler on top of the heatsink, so it's still atleast got contact with it.
Try that actually, had some boards at work that stopped working due to the pressure of the cooler mechanism bending the board too much.
It could be a faulty motherboard you have in all honesty, atleast you know that all of your ram works.
If the Pentium D is Socket 775, then yes it would fit, if it has pins on the under side of the cpu, then no it won't...