The best you can do is a Radeon 4670. It should be fine even with a sub par 430W PSU. This model from Sapphire has a cooler particularly suited to overclocking ithttp://www.newegg.com/Product/Product.aspx?Item=N82E16814102854
You could also go with the GT240 (the GDDR5 version of course), but it's a bit past $80. Anything more powerful would probably require a better PSU than the one you currently have.