A Radeon 5870 remains a very powerful card despite being a generation old, easily outperforms the 4890
, and can be found for about $200... if you can find it. AMD doesn't stock retailers with it anymore, so you would have to buy it from a non-major seller (IE from someone on Ebay, etc.) The 5850 also outperforms the 4890
, if to a lesser degree, but like the 5870 you will probably have trouble finding it in stores.
The 6850 is about equivalent performance, if not slightly more than, to the 4890, and you have the benefits of it being a card in the most recent generation and being in stores. That goes for about $150. The 6870 is another step up (about where the 5850 is compared to the 4890, IIRC) for about $180.