Nope, that doesn't work because all those single PCI-e datalines are running parallel into the PCI-e slot. If you plug a PCI-e 1x into a 8x it doesn't have the pins to connect to the other PCI-e datalines.
DDR2 isn't worth the extra money right now, it isn't faster than comparable DDR memory (I think it's actually slightly slower due to higher latency).
A good Soundcard still beats any onboard sound, I'd say even that high def 8channel audio solution and you can always add a FireWire 800 card ... but you would need a board with 6 PCI slots , the three you already need plus RAID/soundcard/firewire ...
Dunno about the LGA775 board, I would prefer a AMD Athlon64 solution but that's my personal opinion
You are correct, the BIOS has to support running a PCI-e card with less lanes in a bigger PCI-e slot. The german section of THW tested a videocard in a PCI-e 16x slot at 1x, (2x), 4x, 8x and 16x and they had to wait for a new BIOS from the mobo manufacturer (the german test can be found at http://www.de.tomshardware.com/graphic/20041105/index.html ). They taped the connectors so that the card used less of the PCI-e datalines for the slower speeds.