Basically i have 400~$ to spend on a new gpu. I'm looking to upgrade from my 770. I don't care about the brand. I play at 1440p. Is the 390x worth it over an 8gb 290x? Is 8gb even needed? How does the 970 compare? I'd like to play skyrim modded at 60fps at 1440p. Power consumption isn't an issue.
You won't use 8GB for 1440p; the 8gb is going to be useful for ~3-4k and beyond.
970 has only 3.5GB (0.5GB is running much slower) which is why you might be loosing some performance on 1440p due to memory bandwidth after you go over 3.5GB texture buffer.
Getting quality non-ref 290x with 4GB is best option at this moment. You'll spend less cash too, and if it has proper cooling - performance will be comparable if not the same once memory gets OC to 970. (samsung and hynix memory oc's very well - samsung to 6.2GHz, hynix to 6.6GHz)
I'd say 390x 8gb just because modded skyrim usually loves V-Ram, and the 390x has been more competitive with the 980 then 970 due to driver improvements and the faster memory.
Would you go to 8gb on a 290x for 20$ more? Is the 390x really that much better than a 290x for ~100$ more?
no
get a 8GB 290x
390x has better memory chips, better cooling systems.