Hi I a building a website that would have some k2 items shown in a kind of portfolio layout opening in popup lightbox window. What I need is an extension or any kind of advice in order to implement it. I need the whole article shown in lightbox after one of the portfolio items is selected(clicked).

Thanks in advance.

To be more specific cause some people have read but no answers to the topic,
I need advice about the lightbox functionality. Which extension(joomla 3) is good to combine with k2 for this purpose or if there is any oem functionality of k2 and joomla for lightbox viewing items please let me know.

I do not think an extension can do this.
This should be done in your template's overrides.
EG: Take a look at this template which offers modules that open their content in a lightbox www.joomlaworks.net/joomla-templates/commercial-templates/archetype

The Zentools module from Joomlabamboo has an option to open items in Lightbox. It works very well (and is an all around great tool)