Couldn't find a direct comparison between 3850 AGP and 4670 AGP, but this review should give an indication of performance even though it's PCI-E. Keep in mind also the cards here not nearly as CPU bound as your average AGP system is probably going to encounter. Also the 4670 here only used 512 Mb but shouldn't change things a whole lot.
http://www.techpowerup.com/reviews/Sapphire/HD_4650_OC/1.html
Used
1280x1024 for comparison but you can check out other resolutions if you use a larger screen.
Call of Duty 4: -----------3850 - 45.8fps/
4670 - 55.4fps
Call of Juarez:------------
3850 - 28.7fps/4670 - 25.1fps
Company Of Heroes:------3850 - 179.4fps/
4670 - 183.3fps
Crysis:-------------------3850 - 21.3fps/
4670 - 22.2fps
Quake Wars:-------------3850 - 93.7fps/
4670 - 105.3fps
Far Cry:-----------------3850 - 95.1fps/
4670 - 122.3fps
Fear:--------------------
3850 - 89fps/ 4670 - 82fps
Prey:--------------------
3850 - 92.2fps/4670 - 88.4fps
Quake 4:----------------
3850 - 128.9fps/4670 - 93.3fps
Splinter Cell 3:-----------
3850 - 86.8fps/4670 - 82.9fps
Stalker:-----------------
3850 - 53.3fps/4670 - 51.9fps
Team Fortress 2:--------3850 - 61.9fps/
4670 - 68.9fps
Unreal Tournament 3:----
3850 - 100.8fps/4670 - 99.3fps
World in Conflict:--------3850 - 30.0fps/
4670 - 35.0 fps
Folding @ home:----
3850 - 8741/4670 - 7620
3DMark03:----------
3850 - 16620/4670 - 14643
3DMark05:----------
3850 - 12424/4670 - 12202
3DMark06:----------
3850 - 9098/4670 - 7782
Essentially they trade blows under ideal conditions (not CPU bound). The 3850 is a little better under synthetic benchmarks.