I'm curious. How high can I get?
I believe 400 health is max, 20 health per heart and start with 100 health, so 15 crystal hearts.
EDIT: This is current as of v1.0.4
EDIT2: It should be noted that max mana is 200, or 10 mana crystals.
Using Life Crystals, you can get up to 20 hearts or 400 max health.
After you have 400 health and defeat Wall of Flesh to enter Hardmode, you can consume up to 20 Life Fruit to get to 500 max health. Life Fruit are found in the Underground Jungle and give +5 to max health.
You will need 15 Life Crystals and 20 Life Fruit to get to the maximum of 500 health.