The power supply is for the graphics card, not the game. More powerful ones take more power, and it's very possible that yours won't cut it. OEM's (companied like Dell, HP, etc...) often use psu's that are just barely large enough for the system they're in. It's possible that you won't need an upgrade, but it's more likely that you will.
If you can post your computer's model, we can find what your PSU is. Should be somewhere on the front.
(for example, mine is a Dell Optiplex GX240)