Can someone explain me the difference between AGP cards and PCIe? And also tell me what's the better card for gaming at a price of around $250.= (mostly World of Warcraft).
AGP is older, slower, and more expensive technology. The newest cards are PCIe exclusive, and AGP is in the stage of being phased out. If you had the choice between the two, I'd have to say PCIe as you'll receive more options.
The fastest AGP card around is the 7800GS, and usually costs between $240-270, but if you were to go PCIe you could get a card that's twice as fast for the same price, such as the Geforce 7900GTO or Radeon X1900XT 256MB.