I would do it in this order.
1) Upgrade tank size if you need to.
2) Upgrade filtration/equipment
3) New set-up/ New fish ( depending if you have room in current tanks or no)
for instance my husband and I upgraded a few tanks w/ a chunk of our wedding money. then used the rest to add a few more "splurges" to our tanks.
As much as it stinks sometimes to put money into the "neccesities" your fish will always thank you, and one of the fun lessons of growing up/being a productive adult. That and good equipment = more time, money, ect saved long-term.
And of course.. You can always bank it up for something else you really want. Or sit on it until you have a "rainy day".