My suggestion considering you are just trying to play a Disney game is to go with something cheap that will get the job done. Therefore (though dsennete may disagree or at least offer a nVidia alternative) I recommend just getting an old ATI PowerColor Radeon 9250.http://www.newegg.co...N82E16814131284
$50 and it will run cars and probably future games as well (no Battlefield 2 or anything, but future Disney games...maybe).
ON A SIDE NOTE: If you have the money, just buying a cheapo (yet newer) computer may be in your best interest. If the card doesn't help, then it's the processor which may just need an overhaul. If you have a good amount of money, I would recommend just buying a cheap Dell to run your kids games on. My old Dell (1.4Ghz P4, 256MB Ram, Radeon 9600 ran Half-Life 2 okay) was great for a while until I bought this new thing. If you have enough for a basic Dell or something similair, I would suggest doing that instead of dishing out $50 to find out you need a new processor anyway.