First thing I notice Matt is your aperture ... you're going to get lots of noise with a small aperture in low light. Try opening up to at least f/11 or even f/8 for better results
Your DoF will obviously suffer for landscape shots, but if you use a wide angle lens you should get reasonable DoF, if you use hyperfocal focusing. I would think that by shooting landscapes at dusk you'd be after capturing shape + colour rather than detail anyway.
Don't forget that the law of reciprocity fails at shutter speeds of 1 sec or more, so you need to compensate for that by setting a longer shutter speed than is indicated by your camera's meter - which is how come your pic's are quite dark.
Dark areas + dark colours are where you'll get noise.