If this card makes it live, EVERYONE would want to have Purify.
Otherwise, I suggest you follow finkel's suggestion of having the 'X' be equal to the amount of poison counters divided by a certain number (probably 4-5 for the regular version, 3-4 for the upgraded one), and have it round DOWN to prevent confusion. With Arsenic, the various Scorpions, Deadly Poison, and the buff to Pufferfish, this card would have amazing synergy as a whole, and is much harder to counter than Hope (excluding Permanent control).
...you should probably fix the link on your first art image though. It doesn't seem to be working. ^^;