http://www.pugetsystems.com/labs/articles/AutoDesk-Maya-2013-GPU-Acceleration-166/
they have the "pitcairn" based amd hd7870 doing very well in the overall test, even besting the much more expensive flagship(at the time) amd "tahiti gpu" hd7970, while the much less expensive "cape verde" hd7750 coming very close both the higher tier video cards. amd has rebranded many of their cards this year, with the pitcairn based r9-270 and r9-270x replacing the hd7870 series. the amd r9-270x can be found for about $220usd while the slightly slower clocked version, the r9-270 can be found for about $180. the hd7750 isn't really around anymore and it hasn't been replaced by the same architecture but newegg does have some still in stock.... though the hd7770(not shown in the benchmark above) is the same architecture as the hd7750, it just has a few more stream processors and a higher core speed so it would be faster than the hg7750 in maya.
in order of price. the msi hd7770 would be the best bang for your buck in this case.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150663
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127687
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127769
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127761
other searches on the internet show that amd's new "hawaii" architecture is only marginally faster than their other video cards in maya. for about $460 you can get a near flagship r9-290, but it doesn't have any dramatic increase over a $80 hd7750, at least in tests for maya.