Voltage fault or overheated components:
If the PC is functioning but beeps continuously, then there is a problem with a voltage on the PSU being incorrect or the CPU overheating. Enter the BIOS and check CPU temperature\fan speed\voltages.
Also check:
RAM
Video card
Keyboard
Yes, the higher speed RAM will adapt itself to the lower speed RAM.
BIOS beep codes
http://www.pantz.org/hardware/bios/beepcodes.shtml