300W should work.
If you are planning on playing HL2 (release 9/30) then I would definitely upgrade the card... and consider the mobo as well (best to an nForce2). Keep in mind that the K7S5A is based on sis735 which is really three, almost four generations old. I realize that this is supposed to be a budget system but I always think in terms of future, even near future viability. You want to make sure that you're not stuck with a machine you're not happy with.
Edit: one more thing, at the price you are considering for the ATX case and the fans ($45+$12=$57) I would consider getting the Antec SLK3700 case, which comes with a 350W PSU included as well as a rear 120mm fan (the front 120mm fan bracket is empty). newegg is still out of stock on it, but ExcaliberPC has it for $65+$15=$80 shipped.
http://www.excaliberpc.com/product_i...oducts_id=1412