Is it an in-person swap with someone you trust?
Even 3GB is wasted on cards of this speed. The extra 1GB won't do anything. The 770 can, in some situations, be faster. It depends on what games you are playing. I would do it; I like/need nVidia cards. If you've never run an nVidia card, play nVidia-optimized games, or want to use nVidia-exclusive features (that Ansel screenshot tool will be 770-compatible) it could be a good idea.
Bottom line: do it for the experience, not performance.