I don't understand which graphics card is which?

Ok so i planned on getting a GeForce 9800 GTX with a PSU of Corsair VX450W Power supply - 450 Watt.... However when i looked up the Geforce graphics card way to many options appeared, Ones with random letters before the title or some had a plus at the end, and some said 512mb while others said 1g. Now i dont really know much about all these indications so if someone could just help me figure this out before i make a bad purchase i would greatly appreciate it. Thank you!