Try 1 stick on ram in slot 1, then if it still fails, take it out and try the other. It that fails, I would try reseating the CPU, maybe when you were in there you accdientally bumped it a little and now its overheating.. (Its happened to me
If that all fails I'd recommend pulling everything out all PCI slot things (video card, sound cards, etc), unplugging all hard drives, and dvd roms, and then hooking them all back up. Maybe a cable is loose.. I doubt it but its worth a shot. Make sure each side of the ram "clicks" when you are putting it in.
If that all fails, make sure you arn't forgetting any cables, do you have sata drives? Make sure you are only using 1 connector, Either the SATA power connector OR a 4-pin connector.
I hope this helps.