the only thing sata3.0 might effect is when they ise one pci as the gate for it, but this should not effect the pci express slots on the board esp the primary pci-e slot. if you are planning to do a single gpu there shoudl be little impact as most boards even when they do go from x16 to x8/x8 with multiple pci-e gates being used suffer only a 4-5% performance loss and thats with very high end cards such as a 5970 , in a 460 i doubt you'll saturate the x8 line at all
so in short no there shouldn't be any conflicting issues
Yeah the single card will work just fine. CF and SLI are implented differently so you cannot SLI on a crossfire only board, and you cant crossfire on an SLI only board, you need the chipset to support it and AMD chipsets do not support SLI.
yes you will be fine running one care... now you can hack a am3 board for sli but thats a tricky procedure involving steps even most enthusiasts would not be comfortable with and that would needless to say void your warrenty