Re: Upgrading from 5770 to...?
What resolution do you play at? Very few games are limited by the cpu power at 1920x1080 or higher resolutions (Skyrim is one of them, it really benefits from a Sandy Bridge processor), so if i were you i would first upgrade the videocard. The Geforce GTX 560ti is a good option.
If later you want to upgrade your cpu/mobo, i'd suggest you go the intel way (sandy bridge) hands down.