I am thinking about upgrading my old video card and some of the new cards require a certain amount of a power supply. I need to know what my Dell Optiplex Gx240's power supply is.
The supply is probably more than sufficient. Now consider a common problem - specmanship. Many supplies sold to third parties are rated in watts. The 350 watt supply in that Dell would be rated by that third party as 500 watts. Neither has lied. In the game of specmanship, third parties measure something different to make their power appear higher. In an industry where most computer assemblers don't know how electricity works, this specmanship game works.
Wattage says little. Far more important is current for each voltage.
Is the supply sufficient? Nobody can say with certainty. However an indisputable and definitive answer is obtained in only 30 seconds with a 3.5 digit multimeter. A tool that requires so much intelligence as to be sold in Kmart. Or Sears, Lowes, Home Depot, Radio Shack, most any hardware store, etc. Best price probably will be Wal-Mart - less than $18.
With a new video card installed, boot the machine. Multitask to everything. IOW play complex graphics (ie a movie), while downloading from the internet, while searching the hard drive, while playing the soundcard loudly, while something is attached and drawing power from the USB, while ...
Now measure each wire by pushing the red probe into the nylon body where power supply connects to motherboard. Measure any one of orange, red, purple, and yellow wires. Each voltage must exceed 3.23, 4.87, or 11.7 VDC.
If a computer is working just fine, but one of those voltages is low, then that is a perfect example of how power supplies work. Normal is for a defective or undersized supply to boot and run a computer. An example of why only the meter can avoid strange, intermittent or future failures. That one voltage would not provide sufficient current. A perfectly good supply has failed in that system. Then you know which current must be larger on the new supply. Current for each voltage is important – not watts.
But, as I said, that original Dell supply is probably more than sufficient.
For further information about your system, post those voltage numbers here.