Is it worth the extra $50 for the 4th gen i5-4670k or is the i5-3570k still top dog or close enough?
There isn't a lot of difference for gaming at all, about the only significant difference is the built-in video is much better on the 4670, if that is something you would ever use(probably not if you have a dedicated graphics card)
That extra $50 might get you 1-2% better performance in games at most, so you have to decide if that is worth it to you or not.
Check out
Intel Core i5-4670K vs 3570K vs 2500K Gaming-Performance - CPUs - Reviews : ocaholic
But some quick data(all of these on a 7970 vid card)
Bioshock Infinite:
Intel Core i5-4670K Stock 165.79 102.52 %
Intel Core i5-3570K Stock 165.91 102.59 %
Intel Core i5-2500K Stock 161.72 100.00 %
Far Cry 3:
Intel Core i5-4670K Stock 32.2 101.90 %
Intel Core i5-3570K Stock 31.8 100.63 %
Intel Core i5-2500K Stock 31.6 100.00 %
Skyrim(weird results)
Intel Core i5-4670K Stock 108.4 106.27 %
Intel Core i5-3570K Stock 114.6 112.35 %
Intel Core i5-2500K Stock 102.0 100.00 %