Heyyo,
Definately a 6800GT if you have the cash to spare. ATI's are good, but for now their PCIE lineup is overpriced for not preforming so well. If they dropped their prices to be more reasonable, but for now just get an nvidia.
For a dual card setup? I think I'd rather get an ATI Crossfire setup myself as they're more efficient, especially in games that don't support dual vidcards natively. SLI suffers of this as for games without sli support usually get no preformance boost over a single card...