Those cards should work but there is always a change of no video issue with ver2.1 cards in older 1.x slots. Check for BIOS update for the motherboard.
Thanks for your response. My BIOS is up-to-date, released on 3/24/2009 and Intel hasn't updated it after that. I've browsed through forums and people are able to run PCI-E 2.1 cards on this motherboard. So, I should not face issues there. What I would like to know is whether my CPU will "significantly" bottleneck the GPUs mentioned. If so, I would like to downgrade my GPU and save some money for my next PC. I'm interested in playing games like Mafia II, NFS: The Run, Max Payne 3 etc. at 1280x1024 resolution with medium to high settings.