To confirm what others have touched on; the 'Crit Chance' card upgrades the power of your critical hits as opposed to the 'Crit Damage' card which upgrades the *chance* of a critical hit. Now why the hell am I enjoying this so much I don't know.
In order of priority I've been upgrading:
Crit 'Damage' (chance to crit for *all* spells, read the card very carefully) until 100%
Dodge until 100%
Crit 'Chance' (damage of crits for *all* spells)
Damage of spell cards (just so they're uniform and so there's no 'bad draw,' have not been focusing on any one in particular unless it's not dealing as much damage as the others.)
Healing potions until Dodge is at 100%
Defense (because Crits can still miss at 100% chance to crit and if such is the case that means there's a slim but possible non-zero chance of getting hit at 100% dodge. If I can survive that hit and win the fight, that'll be another upgrade in the pile.)
Health upgrades waaaaaay near the bottom because why worry about having more health when you're dodging nigh everything
Crit 'Damage' if the chance to crit is at/over 100% (I was still missing crits every now and again)
Dodge if at/over 100%
Healing Potions if dodge is at/over 100% (because you won't need 'em if you're not getting hit but it'll be nice just in case something does and there's no other cards to play)