For the power supply...
Can you open up the case side panel, look for the (probably) gray power supply box with wires coming out of it, and read the sticker on it? It should give you some info there. You could share the model name/number (or the watts and volts on the 12a rail). Alternately if you computer is a Dell or HP or something, you could share that model, as it may help find the specs online.
For what you were told...
You can play games on Vista fine. I wouldn't worry about that. The video card is the weakest link in the PC.
I think the previous person was just saying that most cheapo $100 video cards can play COD4 because it's an older game now. So you do
not have to pay for a $300 card with high power requirements.
If you can't find the power supply...
You can try a card with low power requirements like:
Gigabyte 6570 (link). It's much better than the X1200.