Hey there, try reseating the memory/cpu heatsink/video card/cables/etc. If that doesn't work, try one stick of memory at a time (or try different memory) and in different slots. If you have another vid card to try, give that a shot too.
I had this happen with an Athlon XP box recently. Dang thing refused to boot. Turns out it was a bad stick of generic DDR...
Good luck
OH