Power but no boot or beeps

Okay,
I was playing World of Warcraft at the time and I got up while I was healing to get a drink. When I returned everything was locked and the sound was extremly scratchy and repeating.. and then I hit the reset button on the front... it just went black and everything whirrd but no beeps or anything... so I clicked off the PSU and waited... turned it back on after awhile and it was the same... no beep.. nothing just the whirr of fans... she ain't talkin to me..

This is what I have:
Alienware A51 Case
Intel Socket 478 Mobo (don't have more info right now)
P4 3.06ghz 800mhz FSB
2x512 2x1024 DDR Ram
512mb AGP Nvidia Card
Sound Blaster Audigy 2
PSU 850w

Now, recently I changed out a few things because my comp was severaly out of date after 4 years.. if you can tell by the socket 478... I got the vid card first... it needed more power so I dropped my 420w PSU for a 850w PSU (was pretty cheap like 250 or so)... it worked great and I also bought 2 1024mb ram sticks... those worked great... everything was fine till the day it died (like two to three weeks after everything went in)... I've tried replacing the ram sticks that I bought with the old ones... I've tried booting it off 1-4 ram sticks in different slots.. I've tried resetting it with taking out the battery for 10 minutes... I even took off my heatsink on my CPU to see if the P4 was damage and then replaced the P4 because I thought I had it figured... I've tried changing out my video card with my old one.. I've tried unhooking my dvd-rom, floppy, and taking out the sound card.. I've tried doing everything above on both my 420w and 850w PSU.. I'm out of ideas and screaming for help. Someone I was talking to said my PSU is too high for the mobo that it probably fried it. Is it my mobo or is it something else?

Thanks

try disconnecting everything thats not required to reach the BIOS, unplug everything to all drives, reset the CMOS (and with the power removed take out the battery and short the terminals), and put one stick of RAM in slot 0, also remove the audio card, and replace anything that you can if you have a known working part (the PSU and video card), you may even want to try booting the mobo with nothing plugged in at all (remove the CPU and RAM, it should beep), if you can get it to beep then try adding back parts to see where it breaks

if it does not get somewhere in the BIOS after that it means dead motherboard or dead CPU, and i know that CPU happens to have a problem with overclocking, if you run it with anything past the stock voltage the internal traces will slowly deteriorate and the CPU will die (though overclocking at stock voltages is fine, i know because i have one)

however to me this sounds like it may be a heat issue, either something overheated and broke or the PSU overheated and had some voltage fluctuations that could have broke some part.

If you can't get it working, then i would say its time for a new computer, there is no use in trying to upgrade that system or getting a replacement, its all maxed out and there is a chance that you will misdiagnose the broken part and be forced to buy a bunch of stuff (a surge from the PSU can break pretty much everything in the system at once)

Thank you for your response.

I already tried what you said in some parts but I went through it again with these results:
Took everything out... every stick of ram, all drives, all cards... each time as I took a piece out I hit the power and got the same result fans whirr'd and the power supply came on... but when I removed the CPU nothing happened.
I already removed the battery to reset it.. I don't know what you mean by shorting the terminals.. so that part I'm still confuzzled.
If you read my post you'll see that I already thought the CPU was the problem so I bought another one (misdiagnosed)... my system is pretty old... but I wasn't overclocking.
Now you say the PSU could of shorted out? Is there anyway to check that before I start buying all new pieces (pretty much everything cept for the PSU and HD).. I just bought the PSU and it worked fine for 3 weeks before everything died.
Also with the heat I have a temp monitor to make sure nothing gets too hot.

So were down to either the mobo or the PSU?

Since either way I have to replace the mobo (probably with a socket 775) and everything else... is there anyway I can check the new PSU before it fries the new system? I'd just get a new PSU but it was kinda expensive... and I had just purchased it a month ago.

Thank you again.

well what brand is the PSU? As for checking it, well you can do simple voltage checks with a voltmeter ($10 at radioshack), but it only tells you if the PSU is truly dead, the numbers you get are not really valid for zero load (though they should not be too far off), to get an accurate measurement you really need to be able to load up the PSU and measure its specs while its under load and running warm, and it can't really be done with cheap equipment unless you want to risk some other computer and are knowledgeable about using the stuff needed to test the PSU

As for the shorting the terminals with the battery, well you can take a paper clip and short the two terminals where the battery goes, shorting it will discharge all the caps in the CMOS circuit instantly and you don't have to wait any period of time after the battery was removed (and thus ensures that you did, in fact, wait long enough)

make sure the system speaker is attached properly...any system should beep if there is a memory failure...so if you remove all the memory modules and you get no beeps...either the speaker is not connected to the mobo correctly or the mobo is toast!