Ok great then:
There are three primary voltage rails inside of the power supply: +3.3V, +5V and +12V. Each of these supplies power to the various components of the computer system. It is the combined total power output of all these lines that make up the total power output of the system. The formula used to do this is:
- Wattage = Voltage * Amperage
So, if you look at a power supply label and it shows that the +12V line supplies 18A of power, that voltage rail can supply a maximum of 216W of power. This may be only a small fraction of say the 450W the power supply is rated at. The maximum output of the +5V and +3.3V rails would then be calculated and added to the overall wattage rating.
12V Rail
The most important voltage rail in a power supply now is the +12V rail. This voltage rail supplies power to the most demanding components including the
processor, drives,
cooling fans and graphics cards. All of these items draw a lot of current and as a result you want to make sure that you purchase a unit that supplies enough current to the +12V rail.
If you are going to keep the existing power supply, then you will need to find a card that would not require more than 22A and the +12v rail.
The problem you are faced with is your 305watt power supply; The peak output rating is the highest amount of power the unit can supply, but this is only for a very brief time.
Units can not continuously supply power at this level and if it attempts to do so will cause problems. You want to find the maximum continuous wattage rating of the power supply. This is the highest amount that the unit can supply stably to the components. Even with this, you want to make sure the maximum wattage rating is higher than you intend to use.
Bottom Line: If you want a high end video card, like the ones you have been looking at a replacement power supply is in order. pci ex cards do need power much more than a AGP card.
If you keep the existing power supply then cards like these will work great:
http://www.newegg.com/Product/Produc...82E16814130030
Otherwise you would have to go to a PCI video card e.g.,
http://www.newegg.com/Product/Produc...82E16814121015
http://www.newegg.com/Product/Produc...82E16814145087