In the process of shifting the product lineup to the Radeon HD 6000 series, AMD rebranded the 5770s to 6770s. So nobody should expect a significant difference. Anyhow Sapphire promises some advantages in the Radeon HD 6770 Flex that HT4U.net is exploring in the review.
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