If you already have the switching supply, go ahead and use it, as it shouldn't really have any effect at all on performance.
If you didn't have one, then I'd say get a toroidal supply. Or, if you don't have the drives yet, get AC drives. Those usually run at very high DC voltages, and would likely provide the best performance.
With any decent modern drive, the drives usually won't draw more than 2/3 the motors rated current, so a 700w power supply should be sufficient. Personally, I'd still go with the 900w, as the prices would usually be similar.