The 4870 is slower than the 285. The 4890 is a bit faster than the 285 at that resolution and overclocks like a beast. They are about $230 after rebate. Should come down $10-20 in the next two weeks. Also the GTX275 if you can find one but its a bit more expensive.
Seriously, where do people get their information from? The GTX285 is faster than the 4890. Granted, you can OC the 4890 a bit more than the GTX285, but out of the box the 285 will smoke the 4890.
I see tons of reviews pitting the 4890 vs the 275, but never the 285 for a reason.
Also, While some people even say that the 275 and 4890 are dead even, one can argue that while the 4890 overclocks better than a GTX275, some companies like EVGA, make 1792MB models . That should meet your gaming needs for a very long time.