Intel's first 4x capable chipset (the 820 "camino") isn't really out yet. Kind of hard to make motherboards without the chips!
VIA also has one on the way that supports 4x I believe (maybe that one's out actually), but it will take some time before the big motherboard makers embrace it, and my guess is it may well be even longer until savvy consumers do!
I don't believe I've ever heard of 6x AGP, you may have a very long wait for that!
Well with the motherboards and Intel's i820 Camino chipset itself basically sent back to the drawing boards, you can't expect to see anything with AGP 4X from Intel at least until September. That leaves Via in a very good position with their KN-133 (PII/PIII) and KX-133 (Athlon) chipsets.
AGP 4X is the next step after AGP 2X for video throughput, thus video cards that can utilize AGP 4X now will see a boost when AGP 4X support is on the motherboard. Cards that can take advantage of this care the nVIDIA-Riva-TNT2/Ultra boards and Matrox's G400 lineup.