Hd4870 vs hd5870

Yes,
5870 is worth upgrading from 4870
It performs good upto 2500x1600
It is an good choice for the price point

However,
You may wanna crossfire 2 5770 to get similar performance at a considerabley lower price point
e.g.
2 5770 HD costs $320-$330 USD,
While
A 5870 costs $400
But you still get an upperhand with 5870.


GooD LucK

Depending on the resolution the 5870 is 70 to 100% faster than the 4870:

http://www.techpowerup.com/reviews/ATI/Radeon_HD_5870/30.html

If your resolution is over 1680x1050 it's worth it. Otherwise it wont make much of a difference.