"1) '4 x 240-pin DIMM, support max. 8GB DDR2 800/667/533 ECC and Non-ECC,un-buffered memory' - What is ECC and does this mean that memory speeds other than those listed won't work?"
ECC has to do with error correction, and is generally only used in servers due to excessive cost. Don't worry about it. Also, don't bother with more than 4 GB physical ram unless you also plan to purchase Vista 64bit.
"2) '6-channel Audio I/O' - This means what?"
It means there are 6 ports on the back for audio; mic and line in, front and rear speaker, maybe a S/PDIF connector, a headphone jack. One of those might be an "aux" jack with configurable functionality, depending on who made the chipset.
"3) 'Integrated ATI Radeon X1250-based graphics' - From my own experience, integrated sound cards can be rather difficult to replace. The computer has a tendency to want to use the integrated card rather than the new one you plugged in. I imagine this problem may show up for graphics cards, too."
Such integrated chipsets are generally easily disabled in the BIOS. This is best done before booting into your OS after/before installing the discrete card.
Integrated sound cards, at least, have improved quite a bit in quality lately, and the one in this mobo will probably suit your needs.
"4) 'Maximum shared memory of 256 MB' - This means?..."
That is how much system ram the integrated video chipset can use. If you disable it and install a discrete card, it will be irrelevant.
"5) 'Supports HDMI Technology with HDCP compliant with max. resolution 1920 x 1080p' - HDMI? HDCP?"
That's the latest fad in the TV/Movie industry; HDMI is basically a combined DVI video cable and some sort of digital audio cable in one. HDCP has to do with the copy protection on Blu-ray and HD-DVD. You don't need to worry about these things unless you want to set the PC up to function as a media center.
"6) '(It may not display 1080p smoothly when playing HD DVD and Blu-ray Disc due to current version player limit)' - That sounds like a rather vague problem. 'Current version player limit'?"
Possibly an issue with the integrated radeon chipset.
"7) 'Supports YPbPr component TV-out with max. resolutions 1280 x 720p and 1920 x 1080i' - Can someone decode that for me, please?"
That is component video output, with three separate cables for different parts of the video signal. It allows higher quality output than the old composite (yellow cable on a VCR) out did. Also not relevant if you aren't setting up a media center.
"8) 'Supports DVI-D with max. resolution 2560x1600 (@ 60Hz)' - 'DVI-D'?"
I think that means dual link DVI; some higher end monitors have two DVI inputs, so they can display even larger resolutions over two cables from the video card.
"9) '1 x S/PDIF output connector' - The acronyms! They confuse!"
S/PDIF is a an optical cable used for digital audio transmission; unless you have a receiver and stereo speakers, you won't be using it.
"10) 'Chassis Intrusion connector' - A rather vague and worrying name..."
Some cases have a little circuit that will send a signal to the motherboard if the case is opened, presumably so the system can send you an email alert if someone opens your case while the system is on... Not really useful, but paranoia sells.
"11) '1 x PCI Express x16
1 x PCI Express x1
2 x PCI' - Only 4 expansion slots? Plus, how do I know if my catweasel card will fit in any of them?"
What does your catweasel card plug into now? I thought you mentioned something about floppy? If it uses an expansion slot, its almost certain to be PCI (I doubt you'd have any ISA slots
).
Cheaper motherboards generally feature fewer expansion slots; with all the integrated stuff these days (sound, lan, lots of sata ports, usb, firewire, etc., etc.), there is less need for 8 PCI slots, so they cut them out to save on costs.