I just bought a video card that requires min 300 watts and I didn't realize that my power supply was only 300 watts. Should I get a new power supply or keep the power supply (which may explode)?

Try it out. If the load is too much your PSU will be very noisy or shut down under load. However, companies usually do over-recommend on power to make sure they don't get sued.

What brand is it? A 300w no-name is much less likely to be enough than a 300w Corsair or Antec.

If you want to give me a (US/CDN) price range and what wattage range you want, I'd be happy to find you a new one on the egg.

It won't hurt to try it out, underpowering won't cause major problems(except maybe with Harddrives), and usually the power requirements are only when you're running at peak power usage, which you almost never do.