If I had a choice between the PSR-550, WK series, or DGX keyboards I personally would take the Casio MZ-2000.. Of course the DGX series gives 76 and 88 keys but neither the PSR-550, WK series or DGX keyboards compare to the features of the MZ-2000.. The MZ has initial and aftertouch, full synth engine with 4 element voice structure, room for 8 registration instant recall settings in one bank, MZ has better action than PSR-550, it has an internal 40,000 note sequencer with the ability to hold 10 user patterns that has a capacity of 20,000 notes.. Also the MZ completely beats the 550, DGX's and WK's in terms of sequencing power. The MZ I think is the better of those choices.. Now with the PSR-1000, 640 and 740.. That depends on what you need.. Of course the PSR-1000 is the newer model, but keep in mind this replaces the PSR-640.. The PSR-1000 does however have a far better sequencer than both the 640 and 740.. Plus the PSR-1000 will allow you to customize your own voices. With the PSR-1000 you only have 32 note poly, and same with the 640.. The 740 can compare closer to the PSR-2000 since the 2000 replaced the 740.. The 740 will give you vocalizer, modulation wheel, drawbars, 12mbs of sound, but it doesn't have the sequencing or voice editing features of the PSR-1000... However the 740 does have 64 voice poly.. If it were me buying one of these I'd choose the PSR-1000, but that's for my personal needs.. What you have to do is decide what you really want from the keyboard.. Every keyboard you've listed will save your voice, transpose settings, and much more in the registrations.. They all do this..... Also the sequencers on the 550, 640 and 740 are all direct disk.. Meaning there is no internal memory for the keyboards song recording.. You have to have a disk in the drive to record.. What it does is use the disk as the memory for your music.. When you finish a track the keyboard will ask you save to disk or not... So it basically saves your tracks directly to disk after you record them... Some people found this to get in the way of song writing, while having to wait for the track to save before moving on.. The PSR-1000 and the MZ-2000 have internal memory and will keep your song in memory if you unplug the board.. Also they MZ-2000 will hold 10 songs in memory.. I think the amount the PSR-1000 holds depends on how much memory you are currently using....
GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.