Yon,
I happened to read an article in computer power user recently that explained it nicely. I'll give you a summation of it.
There are two main differences with AGP and PCI Express. Just because it's the replacement to the AGP standard doesn't mean you should be in any rush to go out and buy one.
1) a PCI Express graphics card will not fit into a AGP slot on your MB as DumDum already stated. Only newer MBs with a PCI express slot support it.
2) PCI express: Has 16 lanes of data and supports 4 GB/sec each direction (to and from the video card) for a total of 8 GB/sec. Now you compare that to the AGP (8 lanes of data) 2 GB/sec data throughput and you immediately see the huge increase. However, according to Nvidia, approximately a quarter of that bandwidth is filled up with data that's necessary to make the bus work. So in essence, you're left with 6 GB/sec.
Furthermore, you shoudl know that not all video games are designed to use the total bandwidth (6 GB/sec). In fact, I don't think there's any games out there currently that are programmed to run that high. Of course, in the future, i'm sure you'll see some gaming houses that will make more and more use of that.
