I have an MSI KT3 Ultra ARU motherboard with a 2x/4x AGP video card slot and an AMD Athlon 3000+ processor. I would like to purchase a video card which just maximizes the capabilities of 4x slot. Many of the new video cards say they are 4x/8x compatible but I imagine that once you get to a point, the 4x connection will slow down the faster cards. What NVIDIA and ATI cards just reach the performance threshold of the 4x slot? I'm obviously not very knowledeable in this respect but it just seems to me that there is no need to purchase a video card that is faster than the performance offered by my 4x AGP slot.
Current video cards barely exceed the bandwidth of a 4x slot, so the fastest video card available will be 98% as fast in a 4x slot as it would be in an 8x slot. As in, you won't notice it being slower.
Move a step down from the fastest and you're not using all the bandwidth of the 4x slot.
Basically, pick the card you want/need/can afford/desire/whatever, and use that card. If you want the performance of a 6800gt, get a 6800gt, even if it is throttled a couple % - because a 6600gt will be slower yet.