I own a 4870 card that is almost equal in performance with the 260. I want to upgrade to a 5850 to get more performace (better than the GTX 285) and better power saving features. For now I wouldnt recommend buying anything from the 2xx or 48xx generations only if you already own one and want to make a SLI or Xfire setup. Maybe my solution is not the best, but my opinion is that newer card generations are more future prof than the old ones.
^ +1. Prices should drop really soon, perhaps within the next few months. GTX 260's will drop to low-mid 100s, and possibly down to $100 flat. Even right now, some are being sold brand new on ebay for around $140-$160. Wait it out, you'll be a lot happier.