The graphics card I want to get is an HIS IceQ Radeon 6950, and its PCI slot is 2.1 x16. If I buy a mobo with a 2.0 x16 slot, will the Radeon still be compatible and work? Or do I need to find a mobo with a 2.1 x16 express slot?
Yes you can use the 6950 in a motherboard with a 2.0 slot and it won't really make a difference. I don't even think there are any 2.1 motherboards out there at the moment.