so I was playing with those numbers and modified them slightly to account for overclocked cards and for canadian dollars (all prices are pre-tax/shipping, retail and available only).

Card table               
card   price $CAD   power (watts)   mh/s   mh/s / $   mh/s / w
5770    $90.09    108   215   2.39   1.99
5830    $129.75    195   300   2.31   1.54
5850    $189.86    200   350   1.84   1.75
5870    $262.49    210   420   1.60   2.00
6750    $114.99    100   165   1.43   1.65
6770    $107.49    120   195   1.81   1.63
6850    $149.99    175   230   1.53   1.31
6870    $160.12    175   310   1.94   1.77
6950    $244.79    220   360   1.47   1.64
6970    $329.99    250   400   1.21   1.60
6990    $745.00    475   800   1.07   1.68


Seems to me that the best deals in terms of performance/$ are: 5770, 5830, 6870.
In terms of performance/watts: 5870, 5770, 6870.

When looking at the best performer overall: 6990, 5870, 6970

Now finding a bunch of 5870s in Canada is a PITA and is most likely done through the used market (where the prices are even better!)