Whatever you get, PCI or AGP, is 32 bit, AGP just lets the video use system memory for textures and such The faster AGP enables this to happen, so the more memory you have on MB, the faster it should be, theoretically.
Although some PCI cards allow this too. AGP sounds great, but in practice, I've seen little improvement over PCI. THE agp bus is great, but very few cards actually use it to full advantage. THe advantage of agp would be a kick ass card with only 2 or 4 mb onboard(supercheap) and use system memory for textures and such. I have a dual pentium pro180Mhz(which in 98 runs as a single) and just bought a Velocity 4400 TnT PCI.
It is faaaast. I think it is the best single slot card. The 2d is great, and the 3d is more than fast enough, and clean tv/out.
I kee preading that if you don't have a super fast P2 , then get the Banshee, but i tested it and the TnT beat it on three lowe end systems we tried.