AMD decided to mix things up a bit when they released the HD6000 series first by releasing the midrange cards first and next by using previous high end naming schemes to describe them. So while the HD5870 was the previous generation "bad-boy" the new HD6870 was nothing more than a docile pony designed to give the nVidia GTX 460 some company.
Any major computer upgrade generally requires a full software reinstall too. No so if you heed the advice in the guide to Upgrading a Motherboard without Reinstalling. PCSTATS Tips