How about Radeon HD 5450?
Another cheap option is to buy some old(er) Dell for under 100 bucks, and upgrade CPU, Graphics and maybe RAM. That's what i did for under 250 usd.
But if you could build i3 for that money, i would go with that - always!
Thanks for your reply. Looks like your idea of buying a dell cpu will do a good deal, anyways will search and see.
Could you please mention few Dell CPU models that work good for OSX.