This really is not much of a surprise since pretty much any AGP system is crippled by the CPU and RAM speeds. The recent tests on Tomshardware of the 3850 showed that with a lot of new games the CPU bottlenecks the 3850 and framerates rarely will go past the 2600 and x1900 due to this.
If I were to upgrade an AGP system I would only get the 2600xt for HD video playback and HDCP in Vista. The only way I could see getting the 3850 is if I had a rare Core 2 Duo board with AGP support or an older socket 939 Athlon 64 X2.