I use SX3 too, i strongly advise that you look into getting Ableton running as a rewire slave within cubase, its great for getting ideas down fast and brainstorming... That should be more than enough synths to get you writing some decent tunes, alot of getting it to sound fat is in learning how to process your chosen sounds, thats where your plugins are gonna come in...
I strongly recommend you get all of these :
Camel space, Camel phat, PSP nitro, Arts acoustic reverb, URS classic console compressors, URS Classic EQ's, URS channel strip, Waves platinum bundle, Ohmforce Experience (all the ohmboyz plugins), Sonalskis complete plugins pack (TBK is great for filtering), Fab Filter volcano, Audio damage replicant, Audio damage Dr. Device...
On top of that little lot is all you will need is time, practice and patience...