What Graphic Card for my Computer?

plz help me out im so frustrated right now...i bought a Geforce 8400 GS and it doesnt fit on my PCI slot because is to big, and look like my motherboard dont have AGP slot T,T i need a graphic card, i dont care which one i just want to enjoy HD videos without lag plzzz tell me something i can buy for my computer (Compaq Presariosr1503wm)