Yesh I work at a computer store so I know quite a bit. (Ino child labore, 5 dollars and hour aint so bad.) Anyways, From my brain I say go with Ati first of all cause you seem to be on a tight budget. And here is some wise advice from ME.
"If you buy something once. Make sure you get somthing good."
Yes I would say aim for the 4870, or 5770. 4870 being more powerful in raw power. But 5770 has more technologies. Both run around 180$ CANADIAN.
Good night sir.