the D-50 is perhaps my favorite sounding digital synth. one thing to consider if you can swing the cost.. a Roland V-Synth XT (or V-Synth keyboard with the VC1 D-50 card). This is by far my favorite synth of all synths. The VC1 turns the V-Synth into a D-50, but one with several more waveforms, easier programming and realtime controls. Then you've got the V-Synth itself, which brings together the JP-8080 and VP-9000 and to me is the most perfect industrial/ambient synth in existence. The Virtual Analog side of it is nice, but it is what it's capable of doing to PCM/sample waveforms that make it really shine.

now you could probably buy a Blofeld and a D-550 for possibly a bit less, but it's worth considering.

Agree with the last post - you might consider the VariOs as well. I bought two on EBay and they are very nice machines.

The K5000 is an excellent synth for pads.... if you have another taste than me

I just got a Korg X3R (heh). Was only £40 and it does some nice pads and strings to be fair. Can probably do better for $600 though I imagine .

Korg Wavestation SR

But seriously, if you know synthesis any synth will do... program dude.