Is it important to publish my article in a journal indexed on Thomson ISI?
If we momentarily disregard from discussions regarding the cons of ISI and Thomson-Reuters who run the service etc. and focus on the usefulness of ISI indexed journals for ones career, the answer is yes, it is important at least to some extent. The importance is in part depending on your discipline since ISI indexing is not evenly distributed over disciplines. But generally speaking the ISI listing means the journal publishes papers that are referenced, which is an indication that it is research of some quality and importance.

This does not mean non-ISI-listed journals are not "honourable". But, when choosing a journal in which to publish, you need to make sure it is read by other in your field so that your research is seen and your ideas known by others. You should make a survey of journals that are of reference and if you find ISI-listed journals there is a reason to consider them if your papers fulfill their quality criteria because they will be  seen as prestigeous references when you are evaluated for a position etc.