It reboots pre-post, in BIOS, in OS (if I am lucky enough to get in) in everything.
I swapped the video card, no dice
I tested each stick of RAM, no dice
I replaced the PSU (upgraded from a 600W to a 700W), no dice
ran chkdsk on hdd, hocked up to another machine, everything looks fine
This leaves me with Mobo or CPU
Now what I THINK it might be is the thermal paste. It was applied when I first built the rig a year ago and I only removed the CPU fan for the first time today. So I was going to next try to get paste and give it a go. Not too keen on buying a new Mobo or CPU.
That sounds like it would be worth a try. It sucks but after the troubleshooting you've done you also might try disconnecting everything from your motherboard and reseating it. Or even taking everything out of there and reseating it in the outside chance something's shorting on the motherboard (maybe not sitting on the risers or wire touching or something weird).
Breadboard with just motherboard, CPU & HSF, case speaker, and PSU. You do have a case speaker installed, right? If not, you really, really need one. If your case or motherboard didn't come with a system speaker, you can buy one here: http://www.cwc-group.com/casp.html
Make sure you plug the CPU power cable in.
I always breadboard a new build. It takes only a few minutes, and you know you are putting good parts in the case.
You can turn on the PC by momentarily shorting the two pins that the case power switch goes to. You should hear a series of long, single beeps indicating memory problems. Silence indicates a problem with (in most likely order) the PSU, motherboard, or CPU.
Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.
If no beeps:
Running fans and drives and motherboard LED's do not necessarily indicate a good PSU. In the absence of a single short beep, they also do not indicate that the system is booting.
At this point, you can sort of check the PSU. Try to borrow a known good PSU of around 550 - 600 watts. That will power just about any system with a single GPU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.
The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.
This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.
If the system beeps:
If it looks like the PSU is good, install a memory stick. Boot. Beep pattern should change to one long and several short beeps indicating a missing graphics card. Silence or long single beeps indicate a problem with the memory.