I wouldn't recommend such an upgrade.
As 17seconds said, it's only about 20% faster. Which isn't that good of a deal, given the price tag of a 780.
And unless you're playing on resolutions higher than 1080p, a 7970 pretty maxes out most games at 60fps (right?).
I would at least wait for the next gen nvidia cards for an upgrade.
But hey, if you got money to burn, I'd say go for it.
Although you might consider a 780Ti then.