You haven't given us ANY info, so I'm trying to keep my answers vague. If you'd like to give us a resolution you play at, a CPU powering the thing, perhaps your ram amount and PSU, etc I could probably be more help.
As the answer depends on your personal value of increased performance, versus your personal value for the cash, it's hard for anyone to provide value advice. Here's THG's "best cards for the money" summary. neither 'won" their category so I guess, to the author at least, puts them about even as "honorable mentions".
In short, I don't think 5870 is worth it. Get 5850 and OC it... w/out increasing the voltage u can get about 830 mhz, now that's pretty close a default 5870 of 850mhz. If you choose to up voltage then your OC 5850 specs are higher than a stock 5870 !!! Of course one can say OC the 5870 and you'll leave the 5850 behind.
The point is, you can save lots of money by going 5850 and OC it to match a stock 5870 by upping the voltage. Therefore similar performance with a cheaper price tag.