Computer trouble

Hello,
well my computer was running fine but needed more ram. so i got some. i shut it down unhooked all power and added more ram. when i cut it back on the power light on the front stays orange the fan runs and the cd drive has power i took the ram back out and put the old ram back in the same thing happend. then i took the ram completly out and the tower beeped 3 times . the light was then green . tried a new power supply but still the same . also tried the eraser on the ram . it's my other computer a gateway 300 se xp