Really I think that from a practical, gaming standpoint that there is little difference between AGP 2x and 4x. Synthetic tests can certainly tell you other things though.
If a game starts to use a lot of AGP texturing it might cause stuttering even with AGP 8x so you'd probably just want a video card with more local memory. 128MB is already quite a bit for games you'd run on a P3.
One thing that does use A LOT of AGP bandwidth is ATI's software-based Truform (anything not Radeon 8500/9100). While the 8500/9100's hardware Truform is a technology meant to reduce bus traffic with more complex geometry, the software based emulation on other cards does the opposite. This will be slower if you are at AGP 2x. But it is also extraordinarily CPU limited so it won't matter with a P3. It will be too slow regardless. You need Core 2 to beat the CPU limits.
It's also possible that you might run into a bottleneck in special cases with a Glide wrapper. Some situations use a lot of bus traffic. It's pretty rare though from what I understand and you might need PCIe to get around that entirely anyway.