HI CXF04,
As Mr Halibut suggests, with your existing CDS 3, the obvious move would be to seek out a good deal of part exchanging your 252 / Supercap in favour of a 552. The difference is not insubstantial, as I discovered when making this move some months ago.
As far as the NDS is concerned, adding a second 555 PS is, as Harry says, well worthwhile, and the resulting sound seems more "live music" if that makes any sense, with an extra layer of detail which previously was missing.
I've since added the Super Lumina speaker cables and interconnect, and it's even more of the same, following a lengthy running in period.
Assuming that you will be planning any upgrades incrementally, which single one did I find made the largest difference?
Well, I'd have to say it was the introduction of the 552 DR, so if you're happy with your CDS 3, (and it is a wonderful player), I'd plump for this.
Maybe next have a home demo of the Super Lumina, though make sure the cables have already been well run in by your dealer.
The NDS and twin power supplies is a magical thing, but it may well prove that you will already be more than happy with the outcome of the first two upgrades.
Hope this helps,
Dave.