Having had a Tamron 28-300, I would point you at a 2 lens solution rather than the single one unless you really don't want to change lenses and will only be printing up to A5. At the short end, barrel distortion is particularly obvious and at the long end, expect lovely fringes round all the high contrast edges. If you can, give the lens a try before you buy it and see what you think of the final results - after all, it's your opinion of them that really counts. IMO, a better (relatively budget) option would be an 18-70mm and a 70-300 (either the older G type Nikkor or the Sigma APO).
As for the D70, great camera. "Only" 6MP but images up to A3 print size from it (with decent lenses) look great. Will AF with almost all Nikon AF fit lenses (check the manual - available on-line at the Nikon site). Like the D40, the D70 is a 6MP, 1.5x crop sensor.