Gaming
Gaming has an extra twist. AMD continue to use the AGP protocol, but Intel has moved on to the architecturally better PCI-Express. NVIDIA is currently still using an AGP-derived PCIe method, effectively adding a bridging chip and not employing, like ATI, a native PCIe design.
Most of the 2,000-mark difference can be attributed to AMD Athlon 64 3500's extra gaming power. In particular, the lobby bandwidth tests show marked performance increases on AMD hardware.
Again, differences are almost exclusively down to processor power.
To carry on a now-familiar theme, AMD's Athlon 64 range are simply faster than equivalent LGA775 processors in gaming-related benchmarks.
Close enough so that you wouldn't 'feel' the difference in fast-action gaming, but large enough to push the initial purchasing decision the way of AMD.