you get no beep or POST screen, no nothing?

well the good news is if you swapped out the PSU for a 100% working AND compatible PSU, and still nothing....your hard drive is probably ok...
plug it into a friends computer as a second drive and recover your files.

dont think its the RAM as it should still try to boot up if the RAM is faulty.

which leaves the options of

bad connection between PSU and motherboard
fried mother board
fried processor
corrupted BIOS


try re-setting the bios by taking the battery off for a couple of mins and putting it back on.