I think GTX 770 is better in performance and a little faster too.
http://www.hwcompare.com/15840/geforce-gtx-770-vs-radeo...
Perhaps, but what game pushes textures and draw distances like modded Skyrim with AA? Even the developers didn't bother putting that detailed textures into the game; it was the work of over dedicated fans with lots of free time.
To answer your questions, right now 2 GB is just about all you need for playing at 1080p and for the foreseeable future. If you had higher resolution or multiple monitors, the answer might be different, but really 2 GB is going to be more than enough. When textures get limited space, I believe they do get migrated to system RAM. System RAM is much, much slower than video RAM, so yes, it will effect framerates and make things slow down. Whether or not it judders depends on the game.
You can max out the memory bandwidth in some ways sometimes, especially with the use of filters (transparency multisampling for example)
I'd say it's unlikely though that you'll encounter one scenario in the foreseeable future that it will necessitate more than 2Gb especially at resolutions lower or equal to 1080p.
I wouldn't recommend a 380W PSU, it will be risky. What would happen if your PSU can't cope with the load:
•bad 3D performance
•crashing games
•spontaneous reset or imminent shutdown of the PC
•freezing during gameplay
•PSU overload can cause it to break down
Here you can see for PSU's :
http://www.eggxpert.com/forums/thread/323050.aspx