PCI-E 1.0 at x16 is equivalent to 2.0 at x8. It would only restrict the most powerful current a small amount. The issue is really compatibility, not bandwidth. Some 1.x boards have issues with 2.1 cards. It isn't always the case but it does happen. If you your motherboard has a semi-recent BIOS update than it likely isn't a problem. Otherwise I would google and try to find people saying they are successfully using 2.1 cards on your specific motherboard to be certain.
Did you ever try this together to see if it worked?
I've got a G41M-GS3 and was going to buy a 2.1 pci express asus ati radeon 5670. Asrock are useless so won't answer. If I just knew someone had some luck with a similar board then I'd just go ahead and give it a blast.