Well really it the thing that really affect video speed is usually the CPU.
You could have a really shitty PCI video card on a 200Mhz machine and nothing would play, put that video card in a 600Mhz machine and you'll likely have no problems playing back at all.
AGP really was meant for letting the video card use system memory for video memory(thus the video card can use as much memory as the system has), which is insanely slow so it's not used. Which is why we see cards out with 128MB of ram on them.
AGP does have other benefits, but it really doesn't play much part with your 2D playback.... KTHULU!!!

so there...
~klinky