5xxx cards are out, the 4xxx cards are the old generation now. The 4870 was $300 when it first came out so now the 5870 is to. Nothing new here. The cost of the left over 4xxx cards will start to climb as they become slim pickin's.
If you are in the market for a new card, I'd go 5xxx all the way.
I got a 1GB 4870 for $125 w/free shipping a couple of months ago. I think I bought at the lowest price ever. I was lucky. The day I wanted to buy happened to be the same day NewEgg had a sale. I wish I could time my stock purchases like this.
The 4870's are on their last roll... So scarce that AMD decided to jack up the prices in effort to produce a bit more cash. At this point the 4870's are even more expensive than a decent GTX 260 so don't waste your cash. Get a 4890 and CF or sell your 4870 and try gettin dual 5750's/5770's....