I think you are asking the wrong place, but to answer you first question, the game does not support DX10, and I hope it doesn't for some time, as for your other questions, I think you current card is fine, Im running a ATI X800pro and have no issues with running this game, but the CPU is used a bit more due to the AI calculations, as for the market becoming flooded with DX10 games, possibly but those supported and published by M$ certainly will be, as for others, they should still support DX9 for a while unless they lose a large market share, from stardock's own research, if they support vista only, they will lose 85% of customers, as vista hasn't really been all that well received
Edit
just realised the rock xtreme 770 is a notebook, the specs seem fine but you will need to install DX9 to play sins