It's like the people in the PCI SIG have lost their freakin' minds.
We poor driver writers (not to mention poor civilians in the the marketplace) are supposed keep track of such esoterica as PCI V2.2, PCI-X 533, and PCI Express x16 and actually understand something about the differences? Like, these guys couldn't have devised names that sound just a little more unique than "PCI-X" and "PCI Express"? I think they shoulda stuck with the name 3GIO while they had it. But noooo... it had to have "PCI" in it, didn't it. Just so we wouldn't get confused, I suppose.
Well, if you're as confused as I am, here's the quick run-down about what these new standards are. I've also provided you a really good link at the bottom of this article to an excellent, readable, paper on PCI Express that I highly recommend you read.
PCI -- Also annoyingly called "Conventional PCI" (like there's an unconventional version?). This is good old 32 and 64 bit PCI that you've come to know and love. The PCI spec has been undergoing a significant set of changes over the years to try to keep pace with speed and system advances. One thing you probably haven't noticed (and don't care about unless you're a hardware guy) is a change from 5V signalling to 3.3V. The spec to which vendors are implementing now is PCI V2.2; There are a PCI V2.3 and PCI V3.0 already defined. Are you asleep yet?
PCI-X -- The PCI SIG likes to call this "High performance, backward compatible PCI for the future" which just means that the PCI SIG is not lacking marketing people. PCI-X uses all the same connectors and stuff as "conventional" PCI. The transfer speed is indicated by the goofy moniker added to the end, as in "PCI-X 66" (which supports a 66MHZ clock rate) or "PCI-X 133" (which supports a 133Mhz clock rate). Because the data is transfered in parallel, either 32 or 64 bits at a time (or even 16 bits at a time, don't ask), this means that PCI-X 133 offers a bandwidth of 1.0GB/second, and PCI-X 533 could offer a bandwidth of 4.3GB/second. That's all theoretical, of course. If anybody ever builds a system that supports PCI-X 533, drop me a line, OK?
PCI Express -- This is an entirely new bus architecture, previously known by the name "3GIO." It's got new connectors and everything. It even defines a new PC Card (PCMCIA) standard called Express Card. How different is PCI Express from stuff that came befoer it? Well, for one thing, it performs serial data transfers and it starts with a base transfer rate of 2.5Gb/second. Data is transfered in packets, and effectively routed via a switch. Transfers are bi-directional, so data can flow to and from a device simultaneously. Since data is switched, more than 1 device can be transfering at the same time.
What's even more fun about PCI Express is that cards can utilize as many as 16 transfers in parallel, thus providing (are you ready for this??) up to 8GB/second total throughput (4GB/second in each direction). These parallelized serial transfers are what are referred to as "PCI Express x8" (pronounced "by eight", by the way) for 8 parallel bit streams or "PCI Express x16" for 16 parallel bit streams. And, here's some cool news: AGP is being replaced with x16 PCI Express connections.
Read This Paper
Obviously, I've only been able to scratch the surface of the differences between these bus standard. If you want to know more, practical, useful, stuff about PCI Express, I stumbled across an excellent white paper by DELL (go figure):
Click here for DELL White Paper on PCI Express
This paper is highly recommended. Seriously. There's almost nothing annoying in it.
And, of course, you can always visit the PCI SIG web site -- which, very annoyingly, will require your company be a member (and you have an account) in order to be able to download any useful documents. I mean, you wouldn't actually want people in the community to be able to read these specs. Oh, hell no, they might become educated then.