What really sounds good together to me are an acoustic guitar and a nice pad underpinning for it. I agree that guitar and piano go together well too, as do nylon string guitar and flute.
I recently got a Roland Boutique series D-05. This is a complete implementation of the Roland D-50 synth, including all the extra card content and full midi implementation, in a small package and very affordable. The D-50 was one of my favorite synths for pads and that sort of thing, and it works well with any midi keyboard. I have it connected to my V-Grand.
www.roland.com/global/products/d-05/
www.sweetwater.com/store/detail/D05
Tony