The 4890 1Gb is the better of the two, but not by a huge amount. You're almost better off buying a 1Gb 4870, as that will have plenty of power. Alternatively, you could wait until the 23rd when ATI will be releasing the 5870 and 5850, both of which will knock the socks off the 48xx cards.
Ati 4870 1Gb, but Wait till Ati's 5 series come out,
and the prices will drop, save yourself a little money
200$ wont be enough to get one of the 5850
you could also...save a little more money and get the 5850 later