I think PEZ nailed it, the FST and the Auricaps are at the top of the list if you are going analog XO.
The outboard (OXO) crossover might be next, with SR-71 and Silver wiring coming in next.
The OXO is also significant if you might want to future proof your 626, so that you can go D-OXO (Digital Outboard Cross Over) at some point.
If you are looking at the 626jr, then the first (and best) upgrade is to the megawoofers.
Hope that isn't confusing

Let me know if you have questions regarding any of those and what they do.