yes its worth it,because you know!whenever new cards come out,people would recommend getting the new gpu cuz they have new names and mostly people say that they have improvements over old series.just tell me how are they?dont they have directx11 support?dont they have eyefinity?dont they have horsepower?arent they ddr5?arent they with good features for a user?how you can call them old?just by name?if hd 6870 beats it in any benchmark,the difference would be just 1 or 2 fps.hd 5850 is a great card and has every thing future proof.its fine for your needs.its more than enough to run any directx11 game.good luck with your decision!!!
Compatibility won't be an issue, it's actually a new build I am putting together. I jsut know the 6xxx series handles tessalation better but was debating whether that justified buying one instead - if the 5850 outperforms the6850 but the 6850 is better at tessalation, does the 6850 actually outperform the 5850 with games using it or does it just end up matching performance as ordinarily it is weaker.
To be honest I may save on the CPU and go AMD 955 instead of Intel i5 2500k and use the £80 on a more powerful card anyway...but I do tend to get mesmerised by new CPU tech which tempts me so much even though I know for gaming I should focus on the GPU more! Besides, a 5850 or similar card is plenty power, especially for MMOs I think (including the new ones coming out)