Is power consumption a concern? if not why not look into older hardware like a fx-8350 or 4790k? I am personally in the process of upgrading my old i5 2500k plex server to a 4770k or 4790k(haven't decided on which yet, but I have both on hand)
A 3570 should have no problem handling 2 transcodes, it should be able to handle 3 1080p transcodes. With that being said, what is your buffer size? unless your starting all streams at once having a larger buffer size will help plex bounce back and forth between the transcodes and keep ahead of where your viewing, its just a band-aid but it helps.
Are you transcoding because of needing lower quality streams because of bandwidth limitations while doing offsite streams? If not and its all local instead id look at finding audio and video containers that play well with your plex clients and do the initial transcode from your media to that format so plex wont have to transcode so much on the fly.
You can use your gtx 970 but last i knew gpu transcoding was still in its early stages and was really inefficient and you couldnt have it help your cpu it was you chose one or the other to do all your transcoding. Things may have gotten better now but its been a good minute since I looked into it.