I am actually building a similar system to yours, if not identitcal. However I am not choosing either of these cards. The best card for around £100 ($200) is the radeon 4850 at the moment. In my opinion you can't go wrong choosing this card.
I'm not sure that it makes a lot of difference at the moment. But the motherboard you have chosen has PCI express x16. You probably want PCI express 2.0 x16 if you want the most out of your card.
However you might want to confirm this with someone else as I am unsure on the performance difference.