Here is a post I put in a forum recently about a guy asking if he should get a new card or another 4870, and here is some relevant info i posted:
"it looks like, surprisingly, 4870s are reaching in the mid to high $90's (USD) range on ebay. (compared to the GTX+ 9800-which came out in the same month, for $100 more, no less-and seems to be had on ebay for under $100 easily.) If you can rely on a brand new video card keeping the same value as the 4870 (and the 4xxx series in general) in 2 years, then selling a card and putting that to a new card is at best a little better than getting getting one GPU and a 2nd later.
In most cases, when a card's value isn't held onto nearly as well, you probably are better off planning for sli/crossfire (assuming you bought a psu that can handle it when the time comes), but in your case, I think you will very much benefit trying to sell you 4870 and buying a new card (somewhere in the $200-$300 range)...I think you could get a 460 or 5830, and if you really look for sales and are open to the the upper range of your budget, you can get a 5850 or 470 for around $250-$270."