Finally found a benchmark and I think my recommendation will still go with a 3850 over a 4670 mainly because the internal memory bus is 2x as large on the 3850 than the 4670 and the drivers for the 4670 are not the best which still gives the 3850 slightly better performance. Usually the higher end cards in a series will perform better than the lower end cards of the next higher series if you catch my drift so an x8xx will most likely outperform an x6xx assuming they are within one series of each other.
here's a benchmark, as you can see almost identical performance and either card you get you will most likely be bottlenecked by your processor so in the end it most likely won't matter as long as you choose either a 3850 or 4670 for agp, because anything else is just an absolute waste of money.
average benchmarks:
http://www.techpowerup.com/reviews/Powercolor/HD_4670/26.html
as you can see they are about the same
"It's the land of the 24 hour party, my fist wrapped firmly around a fifth of bacardi. On the sunset strip's where the orphans play tonight."