PCI Express is the replacement for the present PCI and AGP slots, which is why it has a PCI Express graphics card. AGP has a maximum throughput of 2Gs a second, PCI Express supports 4Gb a second.
The first generation PCI express cards reviews do not show any great increase in capability over the best AGP cards, but you will be ready for any upgrades and AGP x 8 is the end of the line. The X600 itself is an average middle of the road card. Looks OK for what you want to use it for and you can replace it with something better latter.