I think they perform about the same... I'd prefer the 5770. Either way you'd be fine.
It's just an all around great card... low power usage is often mentioned. if they're about the same price and they perform about the same, the 5770 would be the clear choice to me because of it's features, but that's just me. Although, with GTA IV... I would prefer a more powerful card than both, but then again that's just more $.
Also... you can always add another in the future. If you plan on adding another in the NEAR future, I'd just go big now.
As far as the board... it has a 16x PCI-E slot, so I'd assume it would work. What Power supply do you have though?