First combo I found was an XFX Radeon HD 4670 w/ OCZ 500W PSU: http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.470392
$80 after rebates
I also found the same combo only with a 5550:
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.466055
$80 after rebates
If you want a singular card you could grab a 5550 for 50 bucks:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102892
There's also the 5670 for $75:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102870
But in all honesty you should really invest in a new PSU, 300W won't get you very far, whereas anywhere upwards of 400 seems (to me) to offer many more selections and doesn't cost very much.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007657 600014019 4025&IsNodeId=1&name=$25 - $50
Again, I'm not an expert with computers, so take everything I say with a grain of salt, I just went through newegg and found a list of cards cheap.