A strong overclock on the 6300 will balance just fine with any single GPU card, even the 390X (based on the leaked performance numbers).
Used 780's are going for ~$175 on Ebay. If you bought one made by Asus, Gigabyte, or EVGA, you'd have at least a year and a half left on their fully transferable warranty. You could sell your 280X for about the same amount and get a free (albeit slight) upgrade.
Cheers!