Bicycle Mechanics - Headset, Stem, Handlebars Compatability

Hi first time poster in this forum.
What I want to do is replace my road bike's handlebars and stem with a mountain bike set of stem and handlebars. My bike has a 1&1/8 inch headset that is threadless, of make FSA. My handlebars are Nitto B123s.
I want a stem and handlebar of Easton like the EA50 (http://www.eastonbike.com/PRODUCTS/BARS/bar_rise_ea50.html). I want it turn out like the bike on http://velospace.org/node/845.
I'm just confused when the manufacturer gives measurements , I don't what the hell they are refering to.

So in other words do I need a new headset or can I keep my old one?
Oh and can someone recommend a brake handle like the one on the bike I mentioned?
Thanks for your help guys.

You can keep your headset. The issue that counts is that the diameter of the handlebars matches the clamp of the stem. What sort of bike is this?

Thanks man, its a jamie roy single speed. Thats why I thought I should have no problems doing it, its the same frame as the bike I mentioned.

Your headset is likely made for 26.0 bars. A lot of those risers are 25.4. You can buy a shim from Nitto or carefully cut a strip out of a beer can to make up the difference.

Ok, is 25.4 refering to the diameter of the handlebar to stem, or the stem to headset?
Where will the shim go?

25.4 is the measure of the handlebars at the point where you clamp them to the stem (the middle of the bars). The shim would need to go there (the center) if you intend to use them with a stem that is made for 26.0 bars.

What I want to do is replace the handlebars and stem with a mountain bike handlebar and mountain bike stem. Both probably of easton.
However I am wondering will my FSA headset work with the new stem.
Say if I get a http://www.eastonbike.com/PRODUCTS/STEMS/stem_ea50_'06.html, will that be compatible with my headset...

The headset is determined by the fork's steering tube, not the stem. Your current headset is just fine and is completely unrelated to what you want to do. If you want to buy a whole new stem with a 25.4mm clamp, that is your business, but a shim would be much cheaper.

A good starting point for your questions. (http://www.google.com/custom?q=handlebars&sa=Google+Search&cof=LW%3A471%3BL%3Ahttp%3A%2F%2Fsheldonbrown.com%2Fimages%2Fharris-banner.gif%3BLH%3A61%3BAH%3Acenter%3BAWFID%3A3e78bba1cececa56%3B&domains=sheldonbrown.com%3Bsheldonbrown.net%3Bsheldonbrown.org&sitesearch=sheldonbrown.com)