Gigabyte released an Athlon based motherboard that was designed specifically for nVidia 6600GT SLI mode. The performance of the dual 6600GT cards left even the 6800 behind for dead (which at the time was the fastest nVidia card). Since then, nVidia released a dual 6600GT card, which when used in conjunction with SLI gives you 4 GPUs. No single video card chip is yet to beat this configuration. Additionally, while the 6600GT was advertised as a mid range card, it was the only card in its class that also sipported Pixel Shader 3.0 which at the time could only be found on newer high end video cards, not to mention that ATI still only had a 2MB texture cache (whereas nVidia use a 4MB cache). nVidia typically have always had better drivers than ATI. The only issue with the 6600GT is where the resolution is increased to 1280x1024 and upwards where it begins to fall behind the high end cards, but on lower resolutions keeps pace quite well with its more expensive counterparts at less than half the cost.
As far as the market goes though, nVidia and ATI have been pretty even over the last few years, sometimes ATI has been better, but then later it will be nVidia and the top video card had changed places often. Recently though, nVidia has stayed ahead of ATI.
If I was to buy a new video card today, it is pointless however to throw good money after an AGP video card. The AGP slot reached its maximum theoretical limit with 8x the bandwisth of a normal PCI slot (the AGP architecture is actually an extension of PCI). PCIe or PCI Express is a redesigned PCI slot that can continue that development, and comes in different flavours. The PCIe slot used for video cards is the PCIe16 version (that is, equivalent to 16x PCI bandwisth or twice the bandwidth of the best AGP port). PCIe also has provision in its specs for 1x, 2x, 4x, and 8x PCIe slots as well, but very few motherboard manufacturers are actually using them. These slower slots are also physically different (the 1x slot is only about 1/4 of the size of a 16x slot).
In a few years or so, AGP will be totally unheard of just as ISA slots eventually disappeared with the advent of PCI.