hd 3850 is better than 4670 without AA but it is slower and sometimes near 4650 even slower with AA, but in a lot of modern games you cant enable AA so 3850 will be better.
The reason that 3850 runs better in some games is because of CPU bottleneck, which doesn't let either 4650/4670 at their full potentialhttp://www.tomshardware.com/reviews/radeon-4650-agp,2383-10.html
"Unfortunately, with the Radeon HD 3850 still available at a similar price, it's hard to recommend the AGP Radeon HD 4650 until the Radeon HD 3850 is phased out. The Radeon HD 3850 remains the king of AGP game performance, especially for resolutions above 1280x1024.
But since our rig was obviously bottlenecked by the older Athlon 64 X2 3800+ (a fairly typical processor, we think, in a platform armed with AGP graphics), what would happen with a faster CPU? Would that give Gigabyte's Radeon HD 4650 stronger legs on which to stand? In light of the clear CPU bottleneck, we weren't too concerned with overclocking. But with a faster processor helping improve performance, perhaps an overclocked Radeon HD 4650 could serve up more attractive numbers in today's games."