You seem to like the product, don't you ? As for the certification, I would recommend you to take the:
"IBM Certified Database Associate - Informix 11.70 Fundamentals" certification ( #000-558 ) which is easy if you have understood the product.
There is another exam which is "Certified system Adminitrator-Informix" ( # 000-619 ), which is much more complex.
In any case, you will find excellent training material here.
And of course you are very welcome to join our team :-)