If software, go with Propellerhead's Reason 4.0. Too much power and bang for the buck. Countless awards.
I am a hardware keyboard player, but I think Reason is a fantastic product. It's especially awesome because with a powerful laptop computer and one of those 1½ octave keyboards, you can make music at any coffee shop. Pretty killer tools in Reason.
But something you said needs to be addressed, I believe. You stated you want atmospheric sounds and effects. You're really not talking about samples when you say that. Of course, you could, but really not.
Samples virtue is in realism. Emulation of real sounds and instruments.
I think what you are really looking for is great analog sound (with digital processing and FM and wav manipulations stuff thrown in for bonus). (REASON 4.0! has all this stuff, plus sample playback ability).
There are 2 or 3 types of synthesis that are custom tailored for atmospheric sounds with analog being clearly at the top of the list.
DSP - Digital Signal Processing. Very computer processor hungry technology to do a lot of awesome stuff. Think Kurzweil.
Analog - Subtractive synthesis with Filters. Start with tons of fat sound, and filter out the tones and aspects to your taste. Very simple definition, but true at its core. Think most, if not all, synthesis sounds made prior to 1983.
FM - Frequency Modulation (Yamaha DX7, 1983). Also known as Additive Synthesis. Difficult type of synthesis to work with, let alone master. But it introduces some sound abilities worth exploring. It's the opposite of Analog. With FM, you add tonal waves together and manipulate sound by modulating overtones. Hard to explain. Even more difficult to grasp.
There are 2 or 3 others, but hands down, for atmospheric sounds, you'll play with analog and have the most fun. It's easiest to experiement with, and you can always add FM and DSP and Samples to the sound via layering, and have a wonderful time (all by yourself).. until you hit upon a sound that needs to be patented like a Les Paul guitar going through a Martial at volume 10 with distortion. <grin>
Then, make your song, and don't overdo the effects. Ya still gotta make music with good drive and melody.