Game speed problem!

[Eneirda]

can someone help me regarding this problem... i just installed nba 2k9, and the problem is the game is very slow. i mean, i cant play it in high settings. i have to set the crowd to "low" just to play smooth. but if it is set to "medium or "high", the game was to laggy! the movements of the player is very slow!

here is my specs

intel core duo;
e7400 @ 2.80ghz
2gb of ram
geforce 8400 gs 512 mb

what can i do? what part of my hardware needs upgrade? thanks..