It's hard to troubleshoot without a few extra parts.
The store you bought you parts from will other troubleshoot for a very nominal fee - in Toronto usually $25.
Best way if you want to try it yourself is to start from scratch with as simple a setup as possible, take all extras off the MB - disconnect all drives, PCI cards out - just video card and min ram. See if it boots to bios - if yes add one thing at a time until it stops working.
If it comes down to MB vs CPU - 99% of the time it's the MB unless you have had a CPU fan failure.
One quick check for older MBs about the age of your's is look at all the capacitors - if the tops are bulged out - that is the cause.
basically if your not gettin any Beeps from the bios on power up it can only be 1 of 3 things
1st and most probable is the PSU (also cheapest to replace)
2nd is the MOBO
3rd is the Bios chip itself has corrupted (which basically needs new mobo unless you wanna start down the road of BIOS chip replacement)
ok, i know exactly what is wrong with your computer. i've seen this happen all too many times, about 6 or 7 to be exact, and the problem is motherboard failure.
this is due to no post or bios start up, and no mobo activity, there is just power, so if u replace ur psu, and same problems presist, you will need a new mobo.
good time for an upgrade (as an excuse )
but this should solve the problem, cuz hd, mem, gpu, nad pci cards will not cause this problem. im giving it a 96% mobo problem
p.s i had this happen to me about a month ago also, mobo problem