PCI-X runs on 3.3v, and has two speeds. 100mhz, 133mhz. It's a 64-bit parallel bus. The PCI-X slots are usually completely backwards compatible with PCI 64-bit slots (meaning that it has the slot extension to provide 5v to cards that require it).
PCI 64bit is 5v, and also has two speeds. 33mhz, 66mhz. It's a 64-bit parallel bus.
The vast majority of PCI-X cards made these days are "universal" cards. That means you can shove them into a PCI 64bit, or a PCI 32bit slot (both 5v and 3.3v). Same thing goes with PCI 64bit cards, and PCI 32bit cards; it's all a matter of voltage, and the majority of cards are dual-voltage. There are some "older" cards (circa 2002) that are locked to either 3.3v or 5v, and those you have to be careful with.
PCIe is a serial bus. Just like your ethernet port, your USB port, Firewire, IR, Bluetooth, Wifi bus. A parallel bus is kind of "analog", and a serial bus is kind of "digital". So obviously, you can't stuff a PCIe card into a PCI-X slot, nor vice versa.
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On the topic of graphics cards, BFG has lifetime warranties on all their cards. In the case of the Geforce 6800 series of cards, it's highly likely you will burn out your VRM's on the card because Nvidia specified very low-tolerance VRM modules for each line (vanilla, GT, Ultra). They did this to keep their product lines seperate, so people don't have as much success with reflashing a GT card into an Ultra (at 1.4v core). The 5900XT line is notorious for being reflashed to a 5950 Ultra, and matching the card, clock for clock.
ATI still puts high-tolerance VRMs on their cards, and as such, you are far less likely to burn out the VRMs on their cards.
Just something to consider when you're out buying a card. I'd also tell people to stay away from Antec power supplies, as lately their capacitor quality is extremely poor. The supply will do fine for one year, and after that, you have a pretty high risk of burning out components. Such as your 6800/7x00 Nvidia video card, which all require very "clean" power since Nvidia is more interested in keeping their product lines seperate than they are putting robust and high-temperate voltage-regulator-modules on their cards.
Because ATI still puts high-quality, fault tolerant VRMs on their cards, the Radeons can take "dirty" power without burning out.
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