Don't want to step on anyone's toes as I am new here, but I do play the Sims from time to time.
You would have to upgrade your RAM and your video card to play this game.
You need a minimum of 1 Gig for XP and a min. of 1.5 for Vista. You have 1/2 of the required RAM to play this game for XP and 1/3 of the RAM required if you are running Vista.
Acceptable Nvidia cards are:
NVIDIA GeForce series
FX 5900, FX 5950
6200, 6500, 6600, 6800,
7200, 7300, 7600, 7800, 7900, 7950
8400, 8500, 8600, 8800
9600, 9800, GTX 260, GTX 280
I realize it's an expensive game, but it isn't cheap to upgrade these things either and that will also depend on whether you can upgrade your graphics (integrated, etc) card and how many slots you have for RAM, etc....
I would return the game and start with Sims One. You can probably get the game all of the expansion packs for the price you paid for the Sims 3. True, the graphics don't even compare with Sims 3, but your PC won't support Sims 3 at this time. Sorry