Have you measured the voltage across the terminals of the heated bed when it is on? The first thing to check is that you really are getting close to 12V there.
A simple way of getting more power to the heated bed is to use a 12V LED power supply turned up to 14 to 15V. 14V will hive you 36% more power. If you use the same power supply to power the Ramps and you have an LCD or anything else with a significant current draw connected to the 5V rail, then you will need to use an external 5V supply or regulator to avoid overheating the regulator on the Arduino.
ATX psus normally have the 0V outputs connected to mains ground, so they can't be connected in series without modifications.
A simple way of getting more power to the heated bed is to use a 12V LED power supply turned up to 14 to 15V. 14V will hive you 36% more power. If you use the same power supply to power the Ramps and you have an LCD or anything else with a significant current draw connected to the 5V rail, then you will need to use an external 5V supply or regulator to avoid overheating the regulator on the Arduino.
ATX psus normally have the 0V outputs connected to mains ground, so they can't be connected in series without modifications.