CX600M(wires are included) and 600CX are same PSU's. Modular cables are only difference.
If I were you I woud go for Seasonic S12II-620. A little bit pricier but much more quality supply.
About GPU... Since 7970HD is flagship card for ATI in HD7000 series I don't think that it will be arround when new 9000 series comes. Probably you won't be able to buy any 7000 series cards anymore. Depending on your current GPU you should consider buying 7970.
As for cases... I won't be able to help you since i'm FULL TOWER freak and MIDI or simple BIG tower's are not an option for me.
You can google "best MIDI tower cases review" and find something you like.