GTX 260 Core 216's have already begun to drop below $180 right now, and many 1GB versions of the 4870 can also be had around that price. Compare things to Q1 last year when you'd easily pay $200-250 for an 8800GT 512 and you're getting far more bang for your buck.
I'm also uncertain what it is that is making so many people think the 4890 is going to push the prices of these cards down. Since ATI cards typically follow suit numerically with performance (cough... 2K series... cough...), the upcoming 4890 should outperform a 4870 and we should expect it to cost a bit more for that performance. I don't think it'll make the 4870, or for that matter the GTX260, cost any less. I think it will be targeted at the GTX 280/285 in hopes of getting an ATI card's performance similar to (hopefully better than) that of a GTX 280/285 at a price just under those cards. That's what I'd shoot for. It just makes smart business sense to do it that way.
At least we shouldn't have to wait too long to find out.
Also, these are the exact same card models I'm considering replacing my 8800GT with. I honestly don't think I'd be disappointed with either, but since I'm awaiting an RMA, I don't know exactly what card eVGA is going to send me. They don't have any 8800GTs left, so I have to see what they send me before I decide. If I get a 9800GTX or GTX+, I won't be buying either.